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 do hint that you can install a different OS on it:
> Just like any SteamOS device, install your own apps, open a browser, do what you want: It's your PC.
Every other SteamOS device does allow you to install whatever OS on the device, so seems Frame will be the same, judging by that landing page blurb.
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.
I don't see why they wouldn't unlock the bootloader, it wouldn't be the first Qualcomm-based product to allow it and in press interviews they have pressed, quite hard, that the Frame is still a PC.