It checks the key against some kind of authenticator in the superblock, which is what one would expect from any kind of offline crypto. The real problem is tha

It checks the key against some kind of authenticator in the superblock, which is what one would expect from any kind of offline crypto. The real problem is that FDE in the usual sense is (from the strict cryptological sense) very weak encryption, because it cannot afford to expand the encrypted data and has to allow random access (so no random IVs, no authentication tags...). It's relatively trivial to design FDE scheme that is fully secure, but it has 2x block io-op amplification for both read and write, which means unusable performance (certainly on rotating disks, probably even on SSD). Various wide-block encryption modes (AES-XTS etc.) attempt to mitigate attack vectors caused by this, but are secure only under certain FDE-specific attack models. [Edit: formatting and last sentence]