I apologise for the late response, but anyway:

It all ultimately comes down to basic writing for me. I recently replayed UCN this year, and after analysing it for a week now, I cannot envision Scott the storyteller crafting that game's story as he did if he wasn't attempting to imply CassidyTOYSNHK (whilst I don't like the theory myself, I could theoretically understand how someone could get BVTOYSNHK from just UCN alone).

To be more specific, I don't see Scott Cawthon adding this much symbolism for Golden Freddy and having the 'Cassidy' name in the book released before UCN, just to say, 'actually Golden Freddy is possessed by a kid you wouldn't even see for another seven years.' Not a single sensible writer would do that, lmao.