The terms "fake 3D" and "2.5D" in reference to Doom and similar games has nothing to do with sprite based mobs, it's about not having a truly 3D map. Doom's map

The terms "fake 3D" and "2.5D" in reference to Doom and similar games has nothing to do with sprite based mobs, it's about not having a truly 3D map. Doom's maps were drawn in 2D top-down with a height variable for any wall or platform. You couldn't have one room above another, which is why elevators in the game were solid (you couldn't stand under the floor). The various level editors for Doom based games looked like primitive 2D CAD programs. Quake was a whole new paradigm, it was based on a true 3D engine and its maps were generated in three dimensions. You could stand under stairs, under the room above, and underneath lifts.