It does make me wonder.
I opt for the full hand grip instead of just thumb and index, and if the release cord is thick enough, I don't think I can actually throw hard enough to make it slip.
For example, I threw this 590g stone exactly 50m today (I measured the distance accurately with a string), using my 15" Balearic sling.
Video here:
https://youtu.be/0Qpz8_gXCaA(slightly better quality hopefully coming soon)
Didn't experience any slippages, so I think it mostly comes down to the release design and how one grips it.
The stone after retrieval:
I’ve slung stones like that with a 36 inch sling but never with such a short one. That is insane.