2.5D is a technique that defines the artificial depth obtained using visual tricks.
Some games are built on a 2D-compatible engine, then the depth effect is obtained by using multiple layers, parallax scrolling and other tricks or isometric projection. Others use full 3D, but instead of using three-dimensional models and maps, flat sprites and layers are used.
Therefore, games created on a 2D engine that somehow imitate the depth should be allowed. There is no logical reason to exclude them from the competition.