I'm confused by the doubling down here, the bigs all sell cloud storage for precisely this use case. Syncthing might certainly well be better, but I've never heard of it, and billions have heard of iCloud.
It's besides the point: it's certainly not indicative of whether or not it's common sense that iCloud intends to sync documents.
I'm not really doubling down on anything. I personally used Syncthing to keep two computers in sync while I was transitioning to a new one. Paying for cloud storage would have been unnecessary in my case. I did have iCloud before that, but I can't access that anymore ever since my Mac broke.
I do pay for cloud storage to sync my reMarkable 2, mostly because they'll void my warranty if I don't.
It's besides the point: it's certainly not indicative of whether or not it's common sense that iCloud intends to sync documents.