Sounds like a valid antitrust suit... or would be but for the insane court system which ignores most anticompetitive actions.
There's nothing anti-competitive about this. There's no attempt to monopolize or any other aspect that the Sharman act applies.
It's a closed spec which can't be opened, nothing more. There's literally zero anti-trust related issues here, and Valve could in theory use a closed source binary blob driver for HDMI 2.1. They just don't want to (and I applaud them for it).
I know that Slashdotter's knowledge of antitrust laws in general sucks, but something weird has happened in the past couple of months that people suddenly think something not being open source is a violation of antitrust. It's not, this dumb shit has to stop as it distracts from real issues.