Isn’t it pretty obvious not all screen time is the same? If it was then you could do these studies with people staring at simple glowing white rectangles and get identical results.
I do agree that they are different. But they're still screens.
It could very be related to what's on the screen, but it might not. Insufficient data. Screens are very new in relation to the development of human physiology, so I don't think we can assume that screens themselves are not a cause.
The point is, while it might be "pretty obvious that not all screen time is the same," it would also not be that surprising if screen time was still negatively related to the average number of common screen activities over a certain amount of time (which is what you would expect from a study like this).
I'll of course hedge my statement, by saying that the Hacker News community is likely not a representative sample when it comes to common screen activities relative to the general population.
As for whether its obvious: maybe, but there are many cases where something seems obvious (certainly to laypeople in the subject) but that are actually flat out wrong.
So, as someone who hasn't studied these things, I say, yes, it seems obvious, but just because I think its obvious, doesn't necessarily mean its true.