Ah, Count. You must remeber that is only IMDb, and they often misspell/misname/missalign many things. In fact if you wrote to them telling them Rowlf is spelt differently, they would probably change it. And then if you told them it was Rolph they'd probably change it yet again.
I think they are all '(uncredited)' because the film is not yet out, they can't check with the credits. But people like Dorothy have been officialy anounced.