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It would always be possible to project a 3D world onto a 2D plane in any 2D graphics API, which is exactly what the projection matrix does in OpenGL.

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The projection matrix transforms the 3D world into a 3D world in screen space. Two of the dimensions are the screen coordinates, and the third is depth into the screen. The depth is used for Z-buffering (hiding the stuff in back), and for fog and focus effects.

Depth in OpenGL is a separate 2D buffer. You can use it to approximate the effects of 3D space, but it’s still fundamentally a set of 2D operations.

Of course, it’s all fundamentally just bits on a heap, so past a certain point the argument becomes academic.

That, I sort-of agree with. Layering isn't unique to 3D, so it's debatable whether the incorporation of a depth buffer makes OpenGL a 3D API.

It's not layering. It's depth. The depth buffer will work for cases where two objects cross in Z. Unlike the painter's algorithm.

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