That's a gross exaggeration. Every OneNote client (except for the browser one) downloads everything from OneDrive to the local machine and does all the changes to that local copy, which then gets synced to OneDrive. You could take weeks' worth of notes and peruse mountains of reference without any Internet access and the app wouldn't bat an eye.

It's true the app's positioned as an online service rather than a file reader for offline files the way it started, but calling a native app that operates on local data a "thin client" is just disingenuous.