i think that he moreso implied that he was a glitchtrap-induced hallucination than an actual physical object/manifestation (hence why he said that he served a "VERY specific purpose")
I say that because he said said that he wanted to add William back into the story because it "felt right" and he calls him Afton in the Freddy-Files so.....
If this indeed is Afton, then it’s a great villain. Love the idea of Afton being too angry to die. If it’s the Mimic, then, while I like the idea of the Mimic, I don’t like the idea of him replacing Afton as the main villain.