A few thoughts after having read only the abstract:
1. The decline (which is reported to be less frequent in cannabis users) is self-reported subjective cognitive decline based on users’ experience.
2. I think that one of the hallmarks of cannabis use is an immediate decline in short-term memory. I believe this usually reverses itself after some time (hours or days).
3. I have used a lot of cannabis in the past. I stopped entirely - for now - for quite some time and I think my mental acuity is currently at its peak over my lifetime and may be growing.
4. Not only is cannabis a very highly cultivated plant whose potency has been increasing a lot for decades (citation needed), there are a ton of cannabis-derived products out there today that have just a ton of THC, way more than one could get by smoking even the highest potency bud. I think this has prohably changed the game entirely. I used a few of them and I would absolutely recommend caution, far beyond the legitimate caution I would recommend for even smoking cannabis bud.