The statements "the salt will always be known" and "it's different for each password" aren't mutually exclusive. You can have a unique salt for each user / password and still always know the salt for each of those users.
Also, in the case of Wordpress, I imagine the only password an attacker would be interested in would be that of an admin. Presumably you wouldn't be trying to brute force every single users password on a Wordpress installation, anyway. Of course, then again, I'm not sure non-admins have a reason to have an account, anyway, since most Wordpress installs allow unauthenticated users to comment.