First: REM sleep etc. is not a proven cause-effect relation. Second: THC or some other compound might negatively affect that. Third: cognitive decline is broade

First: REM sleep etc. is not a proven cause-effect relation. Second: THC or some other compound might negatively affect that. Third: cognitive decline is broader than memory. Fourth: it was self-reported impression of cognitive decline. I don't think there's reason to expect any relation between REM sleep and the outcome to hold.