I think he meant who performed Rowlf, since that scene had Kermit and Rowlf, two Jim Henson characters. Of course, Jim Henson might have manipulated Rowlf while somebody else manipulated Kermit.
Since Miss Piggy had left before Rowlf was introduced, it's possible that Frank Oz performed him (or Kermit), since he would have been on the set and wouldn't have had a character he needed to perform. Of course, The Muppet Show Fan Club newsletter had an article about The Muppet Movie, saying that it took nearly one day to film each minute (though I think that's exaggerating a bit; It said that it took 87 days to make a 90 minute film, with one minute a day, but then there'd be three extra minutes, and the film was 94 minutes long in it's american release, and 96 minutes long in it's original UK release).
The early script that was posted at Tough Pigs mentions that Statler and Waldorf were originally meant to be in the resturaunt scene. I don't know if their scene was actually filmed and deleted, but if it was, Richard Hunt could have been manipulating Rowlf, or possible performing the hands while Jim or Frank manipulated the body. That would make sense, since all three performers performed other characters and were probably on the set. But then Steve Whitmire might have been performign Rowlf, or possibly Rowlf's hands (since Steve Whitmire usually performed Rowlf's hands after he joined).