you can suggest further experimentation to prove another point, but this is different from assuming a priori that effect WILL happen. The issue is not in the idea itself, but how it is phrased.
They did suggest a new experiment, one lasting longer than 21 days to test their hypothesis.
Hypothesis -> proposed experiment -> results -> questions evoked by results -> new hypothesis -> new proposed experiment
MOST importantly, they didn’t fake any results and went where the data took them. This is the kind of science that has been falling out of fashion for the last few decades in favor of researchers who work based on other principles.
They could have included the obvious (but probably unwanted) alternative hypothesis:
Researchers suggest that these results might mean that changes in cognitive ability could take longer than the study period of 21 days to materialize, or access to devices has a positive effect on attention offsetting the effects of sleep on attention.
It's probably unlikely but it is an obvious possibility.
So is the possibility that attention is an emergent genetic trait. There are many, many alternative explanations. You are lamenting the exclusion of your preferred alternative explanation, but the researchers need to choose one and look further along that branch.
The idea of open science is that other teams would be free to explore plausible alternative hypotheses. Some team might explore yours. Another might dig into my idea about the behavior’s relationship to genetics. And so on.
This is the method by which we move ourselves forward. And it’s easy to see how that effort is hampered by the practice of data tampering and other shenanigans. Which this team did not engage in, even when part of there hypothesis wasn’t supported by their data.
OR - crazy hypothesis - maybe they're familiar with the large amounts of research that already exists which shows that access to devices has a negative effect on attention. Just maybe.
you can suggest further experimentation to prove another point, but this is different from assuming a priori that effect WILL happen. The issue is not in the idea itself, but how it is phrased.