If you add a FIDO2 key as a security key in the admin console, it will show as a "passkey" in Google account settings, but it will actually be a non-resident key used only for 2FA and won't be able to be used for anything more than that.
Keys that do not support resident keys (or when you turn FIDO2 off) show differently in Google account settings which makes it all very confusing. The UX is inexcusable, really.
As a side note, turning on Advanced Protection also turns off passkeys.