That happens with every genre. Genres are labels. You can attach them to works long after the fact, whether the original creator(s) intended or not.

Jules Verne wrote steampunk a century before the term was even coined. Cervantes wrote a novel before novels were a thing (at least in the West), and thereby defined what a novel should look like. The Epic of Gilgamesh predates the Greek tradition of epic poetry by at least a millennium, but nobody denies that it's an epic poem.