Cognitive reserve moderates the effect of COVID-19 on cognition: A systematic review and meta-analysis of individual participant data, 2025, Foreman+

These are not mutually exclusive.

There probably is a significant reduction in CR from the start or very early on. But also a longer-term additional cumulative burden coming from poor quality sleep.

My sleep quality crashed in the very early stages of being sick, and has not improved in the subsequent forty years. The only things that have helped were some nasal surgery to open up the airways, low dose Amitriptyline (10mg), and having a good mattress and a quiet place to sleep undisturbed whenever and for however long required.