Earlier quoted context omitted.
It seems like they plan to support migrating to other services at some point but not yet? Considering the fourth word of the second paragraph is "initially," I think you're correct — this is a work in progress. I hope that Apple will eventually allow bulk downloads of iCloud photos to the desktop. Right now you can only do 1,000 at a time, and it took me almost a week to make a local backup of my wife's iCloud photos…
Easy bulk downloads to the desktop (and hopefully to any attached external drive) would give me real peace of mind. Right now, I am a bit nervous with iCloud being the only full copy of my photos. None of my computers have enough storage to hold all my photos. It would be even cooler if my Synology could download all my photos directly from iCloud.
I'll mention that with the Synology app, you can have your photos be automatically copied to your Synology. I've been doing that since I got an iPhone, so I don't have the need to download from iCloud. Actually, I don't sync my photos to iCloud since I have my local copy.