"Mixed" versus "augmented" reality have no real meaning, which is why there is confusion about which of the two modalities you're referring to.

My question is why avoid the terms overlay, superposition, display and even vision? Why instead purport to have altered reality?

Traditionally in aviation and the military they said "heads up display", because they can't afford to be obtuse. The display allows the pilot literally to keep their head up, instead of pointed down at their instruments. And I'm sure there was no truck for anyone who would have called it mixed or augmented reality.