Cresselia getting the nerf bat is surprising given it has only been like a force on the edge of play since like gen 7 and onward, but I can see the justification if I squint hard enough.
That thing was a menace in Gen 5 VGC and a huge centralizing force, right up there with pre-para nerf, pre-prankster nerf Thundurus and Dragon Gem Draco Meteor back when gems were real. It was just unparalleled support, and I think its fair to say it contributed a good bit to those metagames eventually starring a bunch of stuff that didn't mind speed control constantly. While it hasn't been a problem in a while, it also hasn't been in smaller format without every legendary roaming around ever. The power level of a format like what might be in store for us might be more on par with old formats Cresselia dominated. Maybe whatever competitive play design team GF has (I know they have a little something) got worried now that cress is getting reliable recovery, and might have scared GF into a stat nerf. I'm still a little uncertain this is the actual reason given... Urshifu will exist lol... but that's the best guess I have.
Also I know it's tempting to view things through the lens of Smogon, but there is plenty reason to keep a recovery nerf within the confines of what GF pays closer attention to. I'm not as well verse in BSS as old Gen 5 VGC, but I'm aware stall was a legitimate thing in the restricted 3v3 metas. Quagsire, Toxapex, and Mystical Fire Toxic Eternatus could stall out wins to timer pretty realistically. It's a win condition you could work towards to the point where I believe Theorymon had found a japanese BSS resource with how much time certain actions sucked up for that purpose. This might be an option they took for making games faster while also making sure for things like organized play purposes and their online tournaments, games don't take over a certain threshold of time. Whether it's the correct play for the job, given the splash damage it's done to fan-run formats, I'm not sure. It'll be interesting to see how exactly it'll all come to unfold.