Here's a brief review of the study results. They were only looking for association, not causality:

Long sleep was associated with reduced overall cognitive function (β ± standard error = −0.25 ± 0.07, p < 0.001), with strongest effects in those with depressive symptoms using (−0.74 ± 0.30, p = 0.017) and not using antidepressants (−0.60 ± 0.26, p = 0.024). Weaker but significant effects were observed in those without depressive symptoms (−0.18 ± 0.09, p = 0.044). No significant associations were observed in participants using antidepressants without depressive symptoms.

**Weaker but significant effects were observed in those without depressive symptoms (−0.18 ± 0.09, p = 0.044). **

This is certainly something to investigate further.