It's worth noting that even though Doctor Strange is the Sorcerer Supreme, his fellow human sorcerers, colleagues and foes, are almost always comparable to him for the most part. He definitely edges them out, but it's not like he's leagues apart from them, at least he hasn't been in the recent past. In addition to this, even when he is not the Sorcerer Supreme, he does have additional resources that the others just don't have access to.
If you've followed Loki at all, you'd know that he's an amazing sorcerer in his own right, one of the best on Asgard, if not the best one. Moreover, he has achieved all this by rarely calling upon the mystical powers and principalities, although it is not something that he's forbidden from doing. Calling upon the mystical powers and principalities is something that most human mages do and routinely so. Even if you lowball Loki and place him under the same bracket as Doctor Strange's human sorcerer colleagues and foes, there's at least one thing that Loki definitely outclasses Doctor Strange in, his ability to absorb the cost of magic without consequences due to his godhood, something that is not only explicitly stated in Doctor Strange #384, but is also stated multiple times throughout the issues within the Loki: Sorcerer Supreme mini-series. In fact, beings greater than mortals having the ability to absorb the cost of magic without consequences is an important aspect of the mini-series overall.
Doctor Strange, due to being Sorcerer Supreme, definitely has access to mystical objects and tomes that Loki, or anyone else for that matter, is not privy to. Moreover, being merely exposed to these resources might not guarantee your being able to use them indefinitely, perhaps you need to be the Sorcerer Supreme to be able to use them at all. However, one thing is for sure, Loki wasn't pretending, he was indeed the Sorcerer Supreme during that period. We know for a fact that Loki, as Sorcerer Supreme, held his own against a Doctor Strange that not only had upgraded himself to a god of magic by getting hold of a portion of Yggdrasil's magic, but was also using Sentry to absorb any cost of using that magic that he otherwise would've had to endure himself, thus, freeing him up to use his newfound abilities to their fullest. Facing that sort of an opponent could not have been pretend.
In New Avengers #20 (2013), it was established by Stephen Strange himself that even if someone, who happens to have a nominal understanding of the mystic arts, were to get their hands on a lot of high level mystical resources, they'd still be child's play to him as long as he's willing to pull out all the stops; unless, of course, they're as good as him. He demonstrated this while the then members of Illuminati battled one of Marvel's iteration of the Justice League, The Great Society. Their members were Sun God (Superman), Rider (Batman), Boundless (Flash), Jovian (Martian Manhunter), Doctor Spectrum (Green Lantern), and The Norn (Doctor Fate). This is the degree of dominance that Doctor Strange displayed:
This should be a testament to what sorta sorcerer Loki, even before becoming the Sorcerer Supreme, really is. Even though Stephen has had the experience of participating in the previous two iterations of the Tournament of Sorcerers and winning, Loki is definitely good enough to trick Stephen into thinking that the Tournament of Sorcerers really took place again even though it actually didn't, and that Loki had been chosen to become the new Sorcerer Supreme even though he wasn't participating, let alone win. After having become the Sorcerer Supreme, perhaps he was doing such a good job at it that the Vishanti did not notice, or maybe he had some sorta trickery in place to ensure that they didn't, or perhaps a combination of both. I think we have established that Loki is capable of that. We know for a fact that it's not impossible to trick the Vishanti either. If Doctor Strange can ensure that the Vishanti cannot detect him at all (Doctor Strange: Sorcerer Supreme #49), Loki can at least ensure that they don't notice there being a new yet equally adept Sorcerer Supreme maintaining the balance.
Moreover, in Doctor Strange: Sorcerer Supreme #49 itself, in order to escape his obligation in the War of the Seven Spheres of assisting each and every mystical power and principality that he had pledged allegiance to in exchange for being able to invoke them for aid, Stephen invokes the Emancipation Incantation. Invoking the Emancipation Incantation thereby prohibits him from calling upon the mystical powers and principalities for aid. Participating in the War of the Seven Spheres would've entailed leaving Earth unprotected for 5000 years, which is why he declined.
This made it imperative for him to regain his edge if he was to continue keeping the planet safe. During Doctor Strange: Sorcerer Supreme #80-#90, he managed to achieve exactly that by making himself a powerful sorcerer in his own right, independent of the mystical powers and principalities. He was able to pull this off by honing and mastering chaos magic and the ambient mystical energy of the planet and the universe at large. This became part and parcel of being Sorcerer Supreme from that point on. His newfound status was so impressive that it became all the more crucial for the Vishanti to recruit him by their side. The Vishanti succeeded in doing so after promising Stephen that they'd place him back on Earth at the same exact point in time as the one he'd be leaving with them at. This allowed him to be able to invoke the mystical powers and principalities for aid yet again, presumably because the same was ensured by the Vishanti. Not to the same extent as during his classical days, however, as the fact that he had inconvenienced the other mystical powers and principalities, except for the Vishanti, remained. This is why Doctor Strange isn't as powerful anymore as he used to be during his classical days, this is what brought him closer to his peers. In any case, the abilities that Doctor Strange independently acquired, which went on to become part and parcel of the Sorcerer Supreme moniker, aren't abilities that Loki would be predisposed to by default, so those are definitely some of the abilities that he got access to by being the Sorcerer Supreme, in addition to the exclusive access to the mystical artifacts and tomes that only the Sorcerer Supreme would have access to, that is.
Oh, and it is also confirmed on panel by none other than Doctor Strange himself that he is the one who gave Loki the title and everything that comes with it in Doctor Strange #381.
So yes, Loki was indeed the Sorcerer Supreme then.
While the Tournament of Sorcerers that ended with Loki being declared the new Sorcerer Supreme by the Vishanti was indeed a ruse, the transference of the title of Sorcerer Supreme and everything that comes with it by Doctor Strange to Loki was not. The same transfer was made by Loki to Doctor Strange in Doctor Strange #385, the final issue of the mini-series.
While Loki only handing over the Cloak of Levitation is what's shown on panel, this signifies the behind the scenes transference of the title of Sorcerer Supreme and everything that comes with it, same as how the specifics of the process of Doctor Strange's casting of a spell that contains the name of a mystical principality entailing the drawing of a portion of the principality's powers from the dimension thereof is not shown on panel.
Note: There's an argument to be made that the modern Doctor Strange is finally more or less on par with his classical counterpart in terms of feats, albeit without the degree of plot armour, and the Silver-Age-Superman-like ability to have a unique superpower (spell) for each and every specific scenario faced, that his classical counterpart enjoys. What's also true, however, is the fact that the respective power differentials between the modern Doctor Strange and his peers have not gone back to how they used to be during his classical days. So while the modern Doctor Strange may more or less be on par with his classical counterpart now, his peers are also closer to his classical counterpart today than they happened to be back during his classical days.
