They also explicitly mention os. I'm highly doubtful a product with a Qualcomm soc will support installing an alternative os.
Valve sponsored asahi linux which was a herculean exercise in running another OS on locked down hardware. They've also sponsored wine and fex. It would be a sudden, steep, and unexpected departure for them to go from being leaders in cross platform OS/hardware support to locking down their own hardware platform. It's just not in their nature. They know their nature is good and they know we know it. That's called trust.
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.