They are not the only ones Infinito Wallet give you 12 wallet seed words that cannot be used elsewhere.
You can extract the private key from these wallets
the problem is if the wallet suddenly stops.
That will negate any benefit of HD wallets. They are designed such that anyone can derive the keys given a HD seed. By making the users manually extract each individual private keys, then they might as well not use HD wallets. Having a known derivation path is indeed useful. You can bruteforce the derivation path given a known address and the seeds though.