Not believing that a prompt is an effective way to actually control their behavior is obviously incorrect to anyone who's actually used these things.
It's not a guaranteed way to control their behavior, but you can more than move the needle.
yeah that distinction is pretty important, and in general that guy I believe IS making the point - if you can not control it with guaranteed outcomes - you cannot control it.
The word most relevant to this conversation is “influence.” Influence is possible and users observe it and use it to increase margins of useful outcomes. “Control” is incorrect.