Open source just means you can get the source code, it doesn’t mean you can take over a project.

Asus can’t just take SteamOS, apply some driver tweaks, change some options, and release it as a SteamOS device.

A lot of SteamOS is proprietary, and Valve of course owns all the IP related to the branding. Asus literally needs Valve’s blessing to do it.

Asus are certainly welcome to help Valve with their code, but Valve could also say no this is our project.

And of course they can fork the open part of SteamOS and brand it as something else, and not install steam/steamUI, but that’s half the reason people use the steam deck. It wouldn’t be SteamOS without that.