If you move in 2D it's a 2D game. 2.5D is fine if you really want, but there's no actual functional difference between having 3D models vs pixel art. The gameplay is what defines the game.
You are right that there is no functional difference but 2.5D is an accepted term to define games made to look like 3D or uses 3D models while restricting movement to the 2D plane. It’s not just fine if you really want, it’s a real established descriptor.