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.
Yes, there technically is a Linux kernel, but if it's "just Linux" then macOS is "just FreeBSD", because grep -V tells you so, because it has dtrace, because you run (ran?) Docker with effectively FreeBSD's bhyve, etc.
If you wanna spin it even further neither are Safari and Chrome or any other Webkit browsers just Konqueror because they took the layout engine code from KDE (KHTML).
And you can totally install Debian and even OpenBSD, etc. on a Steam Deck and at least the advertisement seems to indicate it won't be all that different for the VR headset.
Many computer users run a modified version of the GNU system every day, without realizing it. Through a peculiar turn of events, the version of GNU which is widely used today is often called Linux, and many of its users are not aware that it is basically the GNU system, developed by the GNU Project.
There really is a Linux, and these people are using it, but it is just a part of the system they use. Linux is the kernel: the program in the system that allocates the machine's resources to the other programs that you run. The kernel is an essential part of an operating system, but useless by itself; it can only function in the context of a complete operating system. Linux is normally used in combination with the GNU operating system: the whole system is basically GNU with Linux added, or GNU/Linux. All the so-called Linux distributions are really distributions of GNU/Linux!