While it's good that this is finally being provided, it's still somewhat amazing that there isn't any documented API to interact with iCloud.

One can of course, on Apple hardware, use apple proprietary APIs to do some things. Or one can use the iCloudJs stuff from a webpage.

But there's not an official/documented way to, say, write a program that runs on a Linux server to mirror photos in iCloud to disk (or access any other iCloud data).

There are reverse engineered APIs that folks can use to interact with it, but the official iCloud story has been data lock in.