Firefox (only talking about Firefox here) does not use CTAP2, to the best of my knowledge. What you think is "FIDO2" is likely actually U2F/CTAP1. Backwards compatibility with that protocol is part of the FIDO2 spec, but it doesn't enable no-username-provided authentication. LUKS uses libfido2, which supports CTAP2 (and most of CTAP2.1).