CBD on the other hand in "mid range" doses decreases REM sleep latency and (any dose?) also decreases REM sleep suppression related to anxiety. This is something that I imagine needs more study, as there is a huge variability in THC to CBD ratios in legally available cannabis.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8116407

Although most recreational cannabis is high THC : low CBD, there is ever-growing demand for more balanced strains. There's no telling what the average looks like in black and grey markets (although high THC is certainly the default assumption given its intoxicating effects). Before we make blanket statements about the impact of cannabis on sleep, we should probably seek to answer questions like, what is the overall impact on REM sleep with cannabis that has a 4:1 ratio of THC to CBD or lower?

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.

Yeah, that's a real annoyance if you're using cannabis to help manage pain.