This paper is talking about screen use by infants - within the first year of life. Screen interaction can not be the major problem here, it's much more likely to be a lack of interaction with other humans. That's key to infant development, so it shouldn't be a surprise that a lack of stimulus from carers would lead to issues later.
I'm not sure why it would be true that screen time is not a problem for a <1yr old. At about 6-9 months (maybe before) they are able to interact with the screen, and they interact much more with it if it has bright colors and responds to their touch. We would occasionally allow our eldest to use the phone around this age and she would become despondent if it was taken away. So we decided never to do that again.
Of course, a lack of interaction with humans could be a factor too but in our case we were literally playing with it alongside her and she would still get upset if it was taken away.
Not saying that it wouldn't be an issue at older ages as well, since for millenia children learned by watching the other people around them and learning from them, not from screens. That's my essential point - that's utterly essential for infants, even more at that age than later. Not that suddenly it becomes ok!
But if we want to keep the discussion on topic, we should talk about the age group being studied. Beyond that, we want to have a different study.
(edit, grammar because it's really too early in the morning for writing)
I think this might be something to it. I’ve noticed parallels between the stereotypical “kid glued to their phone” and “elderly person glued to slot machine at the casino”. Both of those are more interactive that TV.