I don't think what you say is correct. Just a casual search of the literature reveals fairly detailed descriptions of slavery in pre-agricultural societies. Here's one book dedicated to an analysis of slavery in the hunter-gatherer society in the Pacific Northwest:
You seem to be making a strong distinction between "bands" and "tribes" that doesn't seem to be clear-cut elsewhere, which may be where we're getting confused; maybe it would be useful to just drop that element of my list, and concentrate on the other aspects of hunter-gatherer society?
https://scholar.google.com/scholar_lookup?title=Aboriginal+S...
You can find many more, I hope!
You seem to be making a strong distinction between "bands" and "tribes" that doesn't seem to be clear-cut elsewhere, which may be where we're getting confused; maybe it would be useful to just drop that element of my list, and concentrate on the other aspects of hunter-gatherer society?