But if you're interested in the science of it, it makes sense to distinguish screen time in the abstract, like using a screen for any purpose, from engagement with specific content.

Me, I'd hypothesize little if any difference between the rare encyclopedia-reading 4-year-old and the drivel-watching 4-year-old, as my personal unscientific belief is that it's about socialization with family and peers, not content.

But I'd be super interested in science teasing apart those effects.

If your position is just "most parents suck, who needs evidence?", that's valid too, but IMO less interesting than understanding what's really going on.