That's not at all true. Now people are just copying... whatever band started the prog-sludge trend that Baroness, Mastodon, and a bunch of other bands I can't remember off the top of my head are doing. Was it Neurosis?
Acid bath isn't really a straightforward sludge band, I like them, but they aren't wholly sludge. Then again, neither are a lot of the ones I'm seeing recommended. I could be biased, though, I tend to think of sludge as what happens when black flag meet black sabbath in Louisiana, so it would take a whole lot more southern rock influence to make me think of something as not sludge than it appears to for wolves.