...kind of the point of headcanon that it doesn't necessarily have to follow the original text?
Otherwise it would just be canon.
Speaking as a member of the Touhou fandom, as in, this is how we use the term, headcanon covers anything that isn't either explicitly covered nor denied by canon. In the Touhou fandom, this comes up a lot, as there's not a whole lot of canon information for many characters, so a lot of gaps have to be filled in by headcanon in order to make them a complete character. Making something up like "Harry Potter is actually Indian" wouldn't be headcanon so much as it would be AU, or "alternate universe", the term used for stuff that is contradictory to canon but consistent in its own universe.
Again, just explaining how I use the term "headcanon", and why something like what we see in the cartoon would not be "headcanon" to me.