Back in the day it was probably easy to move goalposts as techniques advanced. Do remember that Duke 3D's Build engine did not draw a true 3D environment by today's standards, and Novalogic's Voxel Space did not use voxels but a heightmap with painter's algorithm.
My personal opinion is that to qualify as 2.5D the game engine must internally use a 3D coordinate system even if the render process is 2D, and 3D requires a true 3D to 3D conversion.
What makes these techniques "pseudo" 3D is that they can only render one very specific type of scene (a road), and even then only under various constraints (road must be in front of the camera, road must continue to the horizon, etc). "Real 3D" typically implies a more general scene renderer.
My personal opinion is that to qualify as 2.5D the game engine must internally use a 3D coordinate system even if the render process is 2D, and 3D requires a true 3D to 3D conversion.