Several mobile GPUs have been reverse-engineered so it's not as hard as you think. As for the ecosystem, if users care enough to pay for it then it doesn't matter how little Apple cares.
Small variants/extensions of mobile GPUs have been shoehorned to work with existing drivers or provide very basic framebuffer capability. This isn't exactly vulkan running on Apple-silicon level of functionality. Maybe Apple's implementation isn't so alien after all and these great folks will pull it off. I hope they do, but I'm still tempering my expectations.
