I'm in the process of actually trying to work out what would be feasible performance-wise if I were to spent the considerable effort to add the features required for base D3D support. It's not looking good, unfortunately. Beyond just "shaders", there are a significant amount of other requirements that even just the OS's window manager needs to function at all. It's all built up on 20+ years of evolving tech and for the normal players in this space (AMD, Nvidia, Intel, Imagination, etc.) it's always been an iterative process.
As I read it, it's just a fun hobby project for them first and foremost and looks like they're intending to write a whole bunch more about how they built it.
It's certainly an impressive piece of work, in particular as they've got the full stack working, a windows driver implementing a custom graphics API and then quake running on top of that. A shame they've not got some DX/GL support but I can certainly understand why they went the custom API route.
I wonder if they'll open source the design?