It uses a master password to generate these passwords, so I actually think you wouldn't be brute-forcing their HN password that way but the master password itse

It uses a master password to generate these passwords, so I actually think you wouldn't be brute-forcing their HN password that way but the master password itself. If you crack one entry, that means you've found the password they're using to generate all of the others. Note that regular password managers also have a master password people can try to brute force, so I actually think this algorithmic method is technically more secure since there's no need to trust a third party to manage the password store, while still having the other flaws that traditional PW managers have.