So, hear me out...

I think Gregory saw Burntrap/The Mimic as Burntrap (in some sense), prior to defeating Princess Quest in OG SB story, and the "Burntrap Ending Drawing" from Ruin.

Although they never diclose Gregory going to the old pizzeria underground, unless you go to complete the burntrap ending... it is, still, possible that Gregory still saw Burntrap, in a sense. Idk how, but they could fit it in as the mimic taking form of scraptrap after FNAF6 fire... but instead of fighting him and the blob and getting the burntrap ending, he release Vanessa with PQ ending, and they both seal the mimic in that room, where he disposes of the Burntrap outfit, and uses the NEW costumes. They aren't all burnup and THAT torn apart.

Also, the blob is renamed to "Tangle" - and in the books, the mimic turns all the ripped animatronic parts into a pile, named "The Tangle ". So, it's possible that the mimic made "The Blob/Tangle", prior to Gregory sealing him up. And naybe Freddy helped seal it up with his voices lines of being down there before. Maybe freddy clear out more of the pizzaplex and they led burntrap deeper into it.

Just a theory. I feel SWS and Scott didnt think about how to turn mimic into burntrap without causing issues with the OG SB ending being princess quest, so they made it a drawing. But its so resembles the ending, he had to have seen it in some way. Maybe he did fight burntrap, and the blob helped remove the mimic endoskeleton from the suit or something idk. Again, just throwing out different thoughts and theories.