> It works for me! However I do vaguely remember having to set them up on Chrome initially, but after that, they now work well even in Firefox. (Only on my computer; on my phone, I get an absurd non sequitur of "security keys only work on desktops").

Oh ok, I haven't tried that. I don't even have Chrome installed anymore.

And yeah I get that message too on my phone, even with standard FIDO2 MFA (not full passwordless Webauthn). Stupid because it does work just fine with other sites.