The developer Mac mini was available since June 2020, right?
Microsoft has been re-writing OneDrive on the Mac side now that Apple finally made its File Provider APIs publicly available. These APIs were supposed have launched on the Mac with Catalina (after being introduced in iOS 11 with the Files app) but they weren’t publicized for Mac until the middle of this year. The result should be better Finder integration since it will use Apple’s own APIs to do so. Microsoft clearly didn’t want to update the existing version for M1 just to turn around and have to update it again for File Provider.And the full MS Office suite was M1 native by December 2020. I'd guess MS prioritised that over OneDrive?