The HIBP database only stores hashes of leaked passwords, but the source material is often (always?) plaintext passwords. If the hash of a password is in the HIBP database, the plaintext password is out there somewhere in a database of a malicious actor.
There are some leaks where passwords are cracked and included in plaintext and there are some leaks where passwords are not cracked and included only as hashes. If the leak includes cracked passwords in plaintext then they will be added to HIBP and can be checked, otherwise they are not included and cannot be checked.
