If Gabe really wanted the money he’d go public

Not necessarily. Going public means he would then have shareholders to answer to. Or maybe he's betting on steam going up in value, maybe taking steam public is his retirement plan. Who knows.

He could have locked down the Steam Deck.

Ehh, sure maybe, but there's probably no financial benefit in doing so. He saved a lot of time and money going with Linux instead of building their own OS from scratch. And because Valve went the open source route, they're free to re-use a ton of open source work, including code licensed under GPL.

And look at Google's Android, much of Android is open source, surely you don't think Larry Page and Sergey Brin are anarchists too?

And you're ignoring the predatory nature of a lot of valve's business. One of the most obvious examples is the CSGO skin cases. Valve is making massive amounts of money off of getting children hooked on gambling.