MixiHoro: SteamOS is open, in that its default settings allow installing arbitrary software; see SteamOS - Wikipedia. I’m not sure I understand your point about operating systems. Epic isn’t capable of building an OS to compete with Windows, or turning Linux into a viable Windows replacement for the hundreds of millions of ordinary PC consumers. Laws exist to regulate monopolies because the argument “if you don’t like our railroad, go build your own” isn’t economically feasible.
Windows remaining open matters deeply to Epic because we’ve been building software and distributing it directly to users on PC for 25 years. PC remaining open is vital to our business and our livelihood, so we’re not going to contribute to a Microsoft effort that could close it down and force all commerce to be intermediated by Microsoft.