I don’t think a canon comic can be considered fanfiction. Even though Coates isn’t the original creator/author, the genre of comic books includes frequently changing writers and artists throughout the years for different issues. This Black Panther is still official within Marvel Comics. Rather than a novel or a solo film which are published once, a comic can be continued as its characters interact within a larger universe, and production changes occur. Comics are supposed to be continued by different people through time – that’s a convention of the genre. No, this version isn’t fanfiction. Coates’ position as a writer working for Marvel Comics means that he is continuing canon, not writing his own interpretations for fun.