Valve has always "let you" install SteamOS on anything you want -- it's copyleft software; they don't have a choice.
What's different now is that Valve will be facilitating you to do it by packaging the software in a less Steam Deck-specific way to make it easier.
I realize some might think this is a pedantic distinction, but it's not okay for the media to disparage Free Software by ascribing more authority to Valve than it actually has.