Im surprised to hear that people never considered D&D to be medieval fantasy. Fantasy genre and subgenres are much better defined now. But where I come from, D&D was the very incarnation of what Medieval Fantasy was.
No one had any expectations of medieval fantasy respecting history; it was fantasy after all. « Fantasy » was basically referring to any fiction that wasn’t realistic or implied to be happening on earth. It was a pretty broad genre with two pretty broad sub-genres of their own; science-fiction and medieval fantasy. There wasn’t much else that was well defined, and almost everything else was described in relation to those two (« a kind of sci-fy but… » or « medieval fantasy except that… »). D&D was exactly what people thought of medieval fantasy; a fantasy that isn’t sci-fi and had wizards, castles, and swords. It really didn’t take much more to be « medieval ».