It's not that different from "real 3d" renderers. Especially in deferred rendering pipelines the rasterizer creates a bunch of buffers for depth map, normal map, color, etc but main shaders are running on those 2d buffers. That's the beauty of it parts operating with 3d triangles are kept simple simple and the expensive lighting shaders run once on flat 2d images with 0 overdraw. The shaders don't care whether normal map buffer came from 3d geometry which was rasterized just now, prerendered some time ago or the mix of 2. And even in forward rendering pipelines the fragment shader is operating on implicit 2d pixels created by vertex shaders and rasterizer from "real 3d" data.

The way I look at it if the input and math in the shader is working with 3d vectors its a 3d shader. Whether there is also a 3d rasterizer is a separate question.

Modern 3d games are exploiting it in many different ways. Prerendering a 3D model from multiple views might sound like cheating but use of imposters is a real technique used by proper 3d engines.