Pretty sure they meant the whole "do one thing, do it well, and prefer composition" part.
But I'm more interested in what parts of systemd don't follow the file metaphor, and what things you think shouldn't follow that metaphor? How would you interact with those things?
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.