Unpopular opinion but correct the whole thing has been designed to lock you to devices they make and have themselves be the arbiter of your authentication.
If that wasn't the intent they could have make the thing work like ssh keys, encrypted at rest, you can take them wherever you want.
It is not Apple's job to implement Passkey into Linux, that is up to the Linux community to build up and figure out, and then for Firefox to implement it on Linux. On the other hand, Chrome just works with it, so maybe Firefox either implements it for Linux in-house, or figures out an existing initiative they could invest resources towards and help to prop up so they can integrate against it on Linux.
I don't think that's the unpopular opinion, I think that's the standard opinion, outside of the corporate security cloister.