Let's be clear here, this is a toy. Beyond being a fun project to work on that could maybe get my foot in the door were I ever to decide to change careers and move into hardware design, this is not going to change the GPU landscape or compete with any of the commercial players. What it might do is pave the way for others to do interesting things in this space. A board with all of the video hardware that you can plug into a computer with all the infrastructure available to play around with accelerating graphics could be a fun, if extremely niche, product. That would also require a significant time and money investment from me, and that's not something I necessarily want to deal with. When this is eventually open-sourced, those who really are interested could make their own boards.
It always baffles me what someone considers "a toy". Remember Linux? Yeah, it was a toy and "never going to be big". GIMP is a homework. I love small projects like this and their potential.
I'm vaguely lead to believe that HDMI's backwards compat with DVI-D (ie why you can have a passive cable from DVI-D to HDMI) means you can use DVI-D signals and an "HDMI-cable compatible" connector. Not sure what this project does however, perhaps it interfaces with pre-made proprietary HDMI logic blocks.
From what I can tell its just that dude didn't code Linux drivers. He's a Windows game graphics Dev so branching out and learning drivers and verilog was his total innovation budget already for the project.