Unfortunately, I think you’re going to run into trouble because fido authenticators are geared towards working as user authenticators rather than as device authenticators.
It certainly should be possible from a technical perspective, but implementation-wise, it’s very likely that the code focuses on making fido devices work with client keys, and using tpms for host keys, since that’s much more focused on headless server functionality.
Oval peg in a round hole.
It wasn’t the crypto key pair part I was referring to, it was the part where fido is geared towards interactive user auth, not non-interactive storage.
It wouldn’t have surprised me if the ssh devs hadn’t put implementing fido support for host keys high in the development list, or that it was tricky to find documentation for. Using something like a tpm is the more typical method.
There’s no technical reason it can’t work, and the op got it to work so clearly the implementation supports it, but that doesn’t mean it’s the most expected setup, which means it might have unexpected gaps in functionality or terrible documentation.