If you actually read that page, you'd realize that "Requirements" is a misnomer, or at least misleading, page title for what's being described.

This article is more of a Troubleshooting page and how to resolve for your specific configuration.

There is not an Ubuntu official support requirement for Proton.

With SteamOS being Arch based, and Valve being a significant Proton contributer, if there were a specific OS requirement, it'd be Arch or AUR capable distro. But that's obviously not the case.