When logging into a passwordless account, the attacker can gain access by inputting more than 256 characters in the password field.
If you disable passwords in SSH, there is no password field. There is no way for you to input anything. The connection will just be refused as the only auth method is an ssh key. If there is no matching key, SSH will refuse the connection.
If you don’t do that, the daemon will fall back to a password, which might then be vulnerable. However, no sane administrator would leave password auth enabled. Usually, you use the SSH password once - to copy an SSH key on the system. Then you disable it.
If you disable passwords in SSH, there is no password field. There is no way for you to input anything. The connection will just be refused as the only auth method is an ssh key. If there is no matching key, SSH will refuse the connection.
If you don’t do that, the daemon will fall back to a password, which might then be vulnerable. However, no sane administrator would leave password auth enabled. Usually, you use the SSH password once - to copy an SSH key on the system. Then you disable it.