At this point, the audience aren't really meant to know it's Fett, although attentive fans have good reason to suspect it and a little out-of-universe knowledge is why so many people are certain.
In canon, we know Boba was a clone of Jango, so anyone played by Temuera Morrison (Jango's actor) could possibly be Boba. We also know that Boba's armour made it out of the sarlacc pit and (if we trust Jabba's threats) that sarlaccs take a millenium to kill, so it's at least plausible that the man himself escaped. He first appears watching Din Djarin just after Din prominently straps the Fett armour to his bike, so storytelling convention suggests he's interested in it.
Outside of canon, Boba Fett's a very popular character and Star Wars, like all long-running fiction, loves to resurrect popular characters. In fact, that's exactly what happened in Legends, where he had a long, exciting, and very marketable life after escaping the pit. And Disney are clearly willing to bring back popular or useful bits of Legends, such as Grand Admiral Thrawn or Palpatine's cloning shenanigans.
Finally and entirely outside of fiction, there have been consistent rumours about Morrison playing Fett going back to at least May of this year, building on fan speculation as early as the first season. In September, a Reddit post noted that a talent agency's online CV for Morrison listed him as Boba Fett in The Mandalorian (it's since been removed).
So the fanbase has been expecting Boba Fett to reappear for a long time, there have been strong indications of it from several sources, and now we have a mystery man who looks a lot like Boba Fett should. I think that's why everyone's so certain, despite the ambiguity of the episode itself.
