So, on AO3 the tag actually is "Alternate Universe - Canon Divergence". So it seems that at least a large part of fandom still considers it a subcategory of AUs. "Canon Divergence" and "Fix-it" (where it does fix things) also work as terms to distinguish it as a different type of AU. I personally do use the different terms because while I like canon divergence fics that are still rooted in the canon, I'm much more picky when it comes to AUs where everything is changed to a completely different setting etc. and those really need to work to win me over. So for me there is a difference.

The Star Trek Mirror Universe is an interesting example here, because the parallel universe is part of the canon, and is definitely a concept across a lot of science fiction.