the experiments in changing school times would never show any benefit.

That's not solid proof. What's relevant is whether different school times can possibly affect things in the longer run, well after the effects of the transition itself are over.

You are just offering folk psychology speculation

"Folk psychology speculation" is a good way to describe the assumption that some teenagers are just "night owls" and cannot possibly manage to retrain their sleep cycle.

'just go to bed earlier' doesn't do anything about the zeitgebers and biology of the thing.

"Just going to bed earlier" encompasses making reasonable efforts that might also involve changing these environmental cues and zeitgebers. Of course if your evening routine involves drinking strong coffee, "just going to bed earlier" might not work very well. The solution is to change that habit.