I'm sorta inclined to agree. Their first The Art of Self Defense makes it pretty obvious that Matt Pike had just finished with Sleep, nice slow sludge. I actually like all their albums, I still think they do unique stuff, and Pike is one of the best in the game. However, there is something about their last two that leaves me cold. The only thing I can really think of though is the producer, Kurt Ballou. It's not that High on Fire play much different, Pike is a little busier, more distorted, but they haven't massively changed their sound that much.
Their first two, the nice heavy thick sounding ones were both produced by Billy Anderson, who produced with Sleep, (early, non world-music)Om, and Mr.Bungle.
worth noting that Blessed Black Wings was the fantastic Steve Albini.
I really think that Kurt Ballou just over mixes the sound, it makes it hard to focus on everything when it's a non-stop assault, the earlier High on Fire work had a lot of breathing room in the mix. Not that any of High on Fires work will win production awards anyways.