@NLTLF
The heck are you talking about. Fan theories never been canon, same as ANY fan works.
If someone posts “My theory is, Spike’s breath is not magical, scrolls are” in separate post, then it’s implied that’s it’s not claiming to be canon, it’s just a theory. On other hand, if someone comments on Spike fan-comic made by someone else, like: “Your fan-comic is WRONG! Spike’s breath is not magical, scrolls are!”, only then it can be said that you’re claiming your headcanon as canon.