Thanks, I have a few 5C's so they should be new enough, I'll need to check my laptop/desktop to make sure SSH is new enough (I run Ubuntu 20.04/Manjaro respectively).
My servers most likely aren't, but I run most of my workloads in Docker or Kubernetes so it's just a matter of time to get them all updated.
However you need fairly modern OpenSSH (this year) for both clients and servers. Both need to be upgraded because the authentication protocol itself is different, so an older server has no idea how to authenticate with FIDO2.
To get the behaviour the parent describes you must make sure to follow the instructions for resident keys, and these instructions won't work on cheaper FIDO (not FIDO2) devices that designed be used as second factors. Without resident keys the authenticator only works when at the computer you used to enrol it, which is fine for a personal workstation/ laptop but not great if you need to roam.