Thanks! I looked into it when that article got posted actually, and near as I can tell that isn't a solution to this problem - and that is fully intentional on Apple's part - because:
1) That article is for things in iCloud Drive, which is everything EXCEPT iCloud Photos. Though you can put photos in iCloud Drive (as files), they just won't be seen as photos, visible in iCloud Photos (or the Photos app), etc. without importing them on an Apple device manually into the Photos app. Where they'll then be synced into iCloud Photos.
The photos that are imported/stored in the Photos App on Apple devices also are not visible in iCloud Drive after being imported into iCloud Photos.
2) It only works on Mac/Apple devices. And only locally. So you'd need to sync to a Mac device, then backup the mac device, then hope it all works. No direct backup is available. So even if it did allow syncing iCloud Photos, it is a really awkward and brittle way to back them up.
3) There ARE APIs for iCloud Drive. But not iCloud Photos.
The web iCloud Photos interfaces also only does manual per-album level downloads (no Takeout or global download equivalent) of photos using the web interface), which stops working at scale VERY quickly.
Notably, Google Photos stopped providing the Google Drive interface to Google Photos shortly after Apple made this their standard operating procedure. So it's a common theme.
Though Google Photos does have APIs and Takeout, so it's lockin is less 'firm', and they're definitely less obnoxious about it. The Takeout data requires some significant massaging to get equivalent from what is visible in Google Photos though.
This is the kind of sneaky trap I've learned to be wary of, as Apple does this a lot for lock-in purposes with their hardware too.