Perhaps there are some sour grapes, but I think more than that he's just hoping for more. Harper clearly has an executes on a grand vision for what PLs should be—he regularly states that there is only one PL and we're just working to slowly uncover it. Haskell fits it in some ways, and now those ways aren't worth talking about any further, and misses it in others. Harper, I believe, writes to galvanize people to move toward his grand vision of unified PL.
ML is better than Haskell in many ways. This should drive Haskell to improve as much as it drives people to check out ML. And it has!