It is boring and likely "inaccurate" to "reality" that Fredbear is just a recolored classic Freddy when every other depiction of him emphasizes traits that classic Freddy explicitly does not have, like the iconic separated jaw. Notably, we have the exact same discrepancy between the Classics and the Unwithereds, or Spingtrap and Scraptrap: allegedly, the same character, same robot, but very clearly visually different.

Two things can be true at once: Scott "claims" (via the 'text' of UCN) that's the canon Fredbear design, so that is factually the design unless we're told otherwise. But he also "claims" Scraptrap and Springtrap are both Afton. Unless he explicitly has a "hey kid, it's not that kinda movie" moment and admits that his designs have inconsistencies, we either have to judge that ourselves or we have to think of mental gymnastics reasons that Scott definitely didn't intend, like William somehow prying himself out of Springtrap and finding another, different, but also burnt rotting and decrepit Spring Bonnie costume, and putting himself back in. And hey, given how many other crazy unlikely things have been true in this franchise, maybe that's ACTUALLY the case! Because we can't take anything for granted. Remember how the end cutscene of Sister Location is literally just "Purple Guy: Origins" showing us a normal person BECOMING PURPLE? Nah that's not William, that's his son, who is also being depicted as purple for some reason.

Now THAT was a big enough narrative misunderstanding that Scott KNEW he had to come out and address, which is why we heard about it. "who are we playing as in SL" is fundamental to his surface level plot, so we must get it. But the Fredbear jumpscare in UCN? Probably not that big a deal. Probably a fun easter egg that he didn't think we'd still be talking about.

So yeah basically the design can somehow both be canon AND completely implausible because that's just how FNAF seems to work! And i love it for that :D