You are right. Somebody night sing nonsense syllables to make a soothing sound and make the song longer. After all, you need to keep singing until the baby is well and truly asleep.

I wouldn't choose those particular syllables myself. They sound too silly. Also, babies are interested in syllables - that's what they listen for when first learning to speak. The crisp syllables of "rack shack bibba boo" will wake the kid up again.

IIRC, some relative of P, P, or M wrote those syllables.