No, that isn't allowing exposure to kernel graphics buffers. That's allowing clients to draw to the main framebuffer with no acceleration at all. If you're memory mapping pixels into user space and drawing with the CPU then you're necessarily leaving kernel space. Around the same time X11 had an extension called DGA that did the same thing. It was removed because it doesn't work correctly when you have hardware acceleration.

So the optimization only makes sense for a machine like yours with no native drivers. With any kind of GPU acceleration it will actually make things much slower. GTK doesn't do this because it would only be useful for that kind of machine running around 25 years ago.