As is almost always the case in scientific studies; it's not a flaw in the study. And this is an incredibly complex confluence of multiple high order systems--parenting, genetics, context, screen exposure to infants--you can't just do one good study/experiment. Too many variables would lead to useless data. Pick ONE and do that, which they did. It's limited info but at least we see correlation from this one (kinda big one).

Edit: and then you get the studies that people deride as useless because they just corroborate something seemingly obvious--ie "study shows parents with ADHD more likely to have children with ADHD." But you combine that with this, another study that says "parents with attention disorders more likely to have children who spend excessive amounts of time on screens" which would call into question whether the original study was merely correlation, or causation. Then have a follow-up study comparing, idk, infants with 2+ hrs of screen time and neurotypical parents to ADHD parents (which then runs the risk of unreliable self-reporting for the parents... and further questions about defining what qualifies as genetic predisposition towards ADHD, and what qualifies as ADHD etc). It gets complicated fast and there will rarely ever be a clear-cut answer, especially when it comes to anything to do with neurology, since we know so little about it currently. But hey that's why we have universities full of research scientists all around the world engaging in scientific dialog and peer review.