Just looking at the table at the bottom of the page, "where can passkeys be used", Ubuntu is the only Linux distro listed, and it does not support Passkeys.
Yes, but it supports WebAuthn, which works fine, so I'm confused as to why this site is Passkeys-specific and doesn't use WebAuthn, which, as far as I can tell, is a superset.
Currently, only platform authenticators are supported to create passkeys with. Support for Security Keys is in development and will be available soon. Certain combinations of Linux distros and browsers will then support passkey creation as well.