The saying is confusing and I would suggest makes the opposite claim. It’s common in sports. You practice at an uncomfortable pace to normalize it, even making mistakes, because if you can’t practice at game speed you won’t be able to compete at game speed. In that context there’s room for both, and I’d say the same for music—you need slow, deliberate practice and also reps in “performance” mode, and it’s probably too reductive to say you should “only” be doing either at any point in time.