If you want to get pedantic the hardware rasterizer in your GPU doesn't rasterize "3D triangles" either. Any notion of '3D' comes from how you project your vertices when transforming them through your coordinate spaces. Though the perspective divide needed to make perspective projection work is baked in, but otherwise NDC is a 2D space. Z only exists for the depth buffer/depth test and perspective correct interpolation. The "shape" of the triangle on screen isn't affected by it.

As long as you can draw 2D triangles there's nothing stopping you from drawing a "3D" scene with a software transform pipeline. Depth sorting gets fun without a z-buffer though.