Agreed that it's much more likely that they'll introduce the X-Men wholesale in a single movie. But dropping hints and foreshadows here and there of other properties isn't unheard of in the MCU -- they're almost obsessed with that sort of thing, especially their after credits scenes.
Me, I'd love to see the established MCU heroes teaming up with one X-Man per movie (i.e. Black Panther and Storm, or Capt. Marvel and Rogue, or Strange and Xavier) until it's time to introduce the team as a whole, but that's extremely unlikely. But teasing through after credits or name drops and such to build them up seems more likely than that idea.