How is orthographic projection used in computer graphics technically classified as a projection?
I think it is more of a terminology shared specifically in the computer graphics community. The transformation $P$ results in a vector one-step-ahead from getting the actual projection. From a 3D position transformed with $P$, we can obtain both

the 2D-projected position in normalized device coordinate (NDC) (by obtaining xy component)
the depth (by obtaining the z component) in NDC

This so-called projection matrix is designed specifically to satisfy the needs in the computer graphics pipeline. The key difference between an ordinary N-D-to-(N-1)-D projection is that the projection matrix results in 2D positions with &quot;depth&quot;, which is required for obtaining a depth map used for many purposes in the computer graphics pipeline.