It's a brand-new version of the app that uses Microsoft's new APIs, and it probably part of a deal to get OneDrive to better use Apple's own file APIs on the Mac and iOS. Maybe the rest of it is old and sucks but the file sync parts are all new.

Some of the dialog boxes look the exact same, but the underlying storage engine is entirely rewritten with the big change being that files are now stored with the same Windows API that backs OneDrive's "Files on Demand" feature where File Explorer now shows the entire contents of iCloud whether or not it is available offline, and Windows will ask iCloud to download files if you try to use them.

How is it unusable? I set it up once and it works perfectly