Tyler probably existed before
Let's ignore the fact that the Klingons hacking into Starfleet records and planting false information1 has terrifying implications for the Starfleet-Klingon war. On that basis alone I'd feel comfortable saying that Tyler was a real person, because the alternative is just too improbable.
In a 2018 interview with IGN, Shazad Latif (Tyler's actor) believes that Tyler was real:
"Yeah, I think L'Rell used a captured soldier," Latif told me, referring to Voq's Klingon partner (played by Mary Chieffo). "She took this guy and then used his body and his brain and his memories, with all the skill she has as a conscious[ness]-transferring scientist. It’s amazing."
He goes on to mention that this was the original intention of the production crew, but of course ideas can change over time (emphasis mine):
"It was an ongoing conversation from very early, because, you know, stuff changes completely in 10 months as you write," he said. "The writers end up changing things to each actor, and they're writing scenes for them specifically. And everything changes sort of organically as you're going along. So ideas that were originally there in episode one might have completely changed, I think. So there was always different people's views, or producers -- you know, different people going, maybe this or this. And, everyone wants to understand the science of it anyway. But, yeah, that was the basic essence of it, that it was more like how it's done. But originally the idea was the same, that he was laid over the top of this core Voq body, and slight, certain organs were put in. And then the transfer of consciousness and memory."
Whether or not this will be addressed on the show remains to be seen.
1 I've seen it supposed that, because Lorca's obvious underhandedness throughout the show thus far, we may not be able to draw any meaningful conclusions from his questioning Tyler; we know that Lorca's lied about things before, and he seems to be keeping some things close to his chest even now, so who's to say he's not lying about Tyler's record?
While an interesting theory, it ignores the fact that many other people in Starfleet have a vested interest in learning more about Discovery's newest crewmate. If nothing else we'd expect Doctor Culber to have pulled his medical records in order to treat him.