Maybe nothing, but can you afford to prove that in court?
THIS ISN'T AN IP/PATENT ISSUE!
This is branding and marketing issue. Anyone can implement the spec, it doesn't need to be a cleanroom implementation. It's almost certain that you could license the patents from the patent holders because HDMI doesn't develop it's own patentable stuff, they just get it from Sony, Panasonic, etc.
THIS IS A MARKETING / BRANDING ISSUE.
Saying they don't want an open source implementation is just a smokescreen. 99% of the implementation is in hardware anyway.
But if a hobbyist were to sell an unlicensed HDMI 2.1 box then the IP holder would likely go after them.
In their eyes, in that case, the IP is being pirated.
This is very similar to h.264 however however in that case the standard is public, commercial use requires paying a fee. Licensing of the HDMI 2.1 specification requires an NDA for specification testing that Valve is not able to perform in order to say that it is a HDMI 2.1 compliant system. They would be running afoul of the HDMI org’s licensing terms.