You are mentioning differential sync. Change notes in Apple store also talk about "documents open ready for real-time collaboration".
It is not really clear: I would assume, if I am online, but open a synced doc on my OneDrive (locally), it will open in AutoSave mode ON, and it would allow co-authoring in real time as if I opened it from SharePoint or OneDrive Online.
Currently when I work on a file on the Mac and open the same file on my iPad with PPT-iOS either one change was overwritten by the latest saver (I had this once, but could not re-build) or - the rule - a duplicate with machine name attached is created - so no co-authoring while working on local OneDrive files.
Is this meant to be covered with latest versions (PPT MAC 16.12, OneDrive Mac 18.065.0329.0008, PPT-iOS 2.12, OneDrive iOS 10.14.8)?