Earlier quoted context omitted.
At least the WebAuthN standard seems to be moving in a different direction [1], which is also surprising to me. In a nutshell, it will be possible for relying parties (i.e. websites) to detect multi-device/backup capable authenticators if required, but disabling multi-device functionality would require a very explicit opt-out, not an opt-in, on the relying party's side. [1] https://github.com/w3c/webauthn/issues/1714
That seems to make fido just a non human readable/rememberable account/password. A somewhat downgrade from original hardware enforced implementation. But also make it more usable to majority of people, because keep something without losing it is just a pain to many people(where is my fxxking key goes again?). And it is still 1000x better than people using same password on every website.
Assuming your browser isn't itself compromised, it is impossible to authenticate using a key for service.com on evil.com. Passwords can't do that. (PAKEs or other challenge/response protocols theoretically could, if implemented in the browser and not on websites, but that's a different story.)