I felt a desire to argue against this quote, but of course a better idea would be to ask what it means.

I'm guessing that "practice" means "the way people are solving this problem now," while "theory" means "the study of what makes a problem-solving method good."

If theorists invent some method that they think is good, but which has already been rejected by practitioners, then I would guess that the theorists have a wrong notion of "good," and they should update their theory on the evidence. If the theorists invent a new method, then there is a chance that it is an improvement, and it may catch on.