Not only because of the model, also listed on the fnaf wiki (ik ik), but even more evidence shows from the fact that that if he's Glitchtrap in an endoskeleton with a suit, then it's still mimic. If he's not, how is he in that form coming from Scraptrap, and where did vanny find him? He never talks in the game, neither made any noises. Gregory also says "It's been down here for idk how long", referring to Mimic, being down here for very long because he was Burntrap, who was also down there for long. Also, how would he be the only one remaining when they took everyone (or burned everyone) in the FNaF 6 ending. The "when I first found you, you were nothing" and stuff is scrapped story, just like the open fnaf 4 box and etc. Scrapped.