About the only site I've come across that still only limits a single authenticator is the only one you've already mentioned. If I pointed out a site that only allowed five character upper case passwords with no lockout policy and really fast responses is that also proof passwords are completely untenable? Or just one actor with poor policy decisions And in the end one can just choose to not use passkeys with sites like PayPal. But the extreme majority I've used allowed multiple, as that's what the specifications recommended.
In the end you can use passkeys without involving Google or Microsoft or Apple at all. Any argument that passkeys lock you into their platforms isn't based in reality and are repeating untruths. You don't need to use them to use passkeys.
https://aws.amazon.com/blogs/security/you-can-now-assign-mul...
It's entirely possible that many sites shared this flaw.
Adoption is really slow. But yeah ok, if multiple are allowed then yes it's no longer a problem. I'm sure I read of more sites that had this problem but it is indeed possible they're fixed now.
Other implementations are often blocked, eg on paypal I've never been able to do it and they also only allow one.
The big tech ones don't have these issues but I won't use them because I want to keep control.