that doesn't in any way mean you can install an alternate OS. But i get your point that at least you can run Arch stuff. Isnt Arch ARM support unofficial? (its been ages since i tried) You dont hear of people running it on RPis for example
They're being a little vague about it but this collaboration to improve Arch's build service/infrastructure is being done in part to faciliate support of multiple architectures.
iirc it was in Tested coverage that Valve said the hardware supports other OSes. It'd be out of character for Valve not to allow for this.
Well. It doesn't say in any docs or specs, but for what it's worth, Valve's hardware has always been open like that. You're free to install windows on your steam deck, for example.