As far a I can understand the study design, it can establish a correlation but bot a causal relationship.
So, instead of implying that using computer up to "2.4 h/day" decreases risk of dementia, but higher screen time increases it, we could as well conclude that dementia patients are either unable to use a PC (< 2.4 h/day) or unable to stop using a PC (> 2.4 h/day).
I'm not confident at all at my ability to interpret this study, since it is definitely not my area of expertise, so, please, correct me if I'm wrong.
Update #1, after reading the article more carefully:
> "First, as an observational cohort study, causal relationships cannot be inferred; observed associations may reflect residual or unmeasured confounding and reverse causation."
that's what I said
> Second, a single baseline self-report likely introduced measurement error, missed secular shifts in digital-media use, and lacked content/context; session fragmentation, breaks, concurrent media, timing (e.g., evening vs daytime), and device type were unavailable.
How much do we really trust a dementia patient's self-report on device usage? I wouldn't even trust a healthy person's self-report. I think that anyone who ever tried to track screen time on computer or on the phone can understand how unreliable are your intuitive guesstimates compared to actual measurements.