Singing uses a different area of the brain to normal speech, and relies on another external timing mechanism as opposed to normal speech, which uses an internal timing mechanism, that’s obviously “faulty” in the stuttering brain. That’s why reading or talking in sync with a metronome or even another person, you’re much more fluent. When you sing, you’re relying on the timing mechanism of the song, NOT on the internal timing mechanism of the basal ganglia.