Technopeasant 2.5D exactly applies to both gameplay (2D) and graphics (3D). There's no such thing as 2.5D graphics (that would be pseudo-3D) or 2.5D gameplay.
And quite generally I don't think first-person counts as 2D gameplay, no matter the trickery of the implementation. 2D gameplay needs things like fog-of-way or disappearing walls to hide things from the player. 3D Maze is not 2.5D Maze.
The term 2.5D discerns 3D platformers from "regular" platformers with 3D graphics.