Respondido em janeiro de 2018: aparentemente agora, finalmente :
Do artigo:
Real-time collaboration is long overdue in Office for Mac. Users have been calling for it for quite some time. A major selling point of Google Docs and several other Office alternatives, it has been a slow rollout for this feature in Office regardless of platform. Limited live collaboration was part of the Office 2016 update, but Excel for Windows, for example, didn't get true real-time collaboration until a beta last year.
Now, users on Mac and Windows can see each other's changes in real-time. As in Google docs, thumbnails show which users are collaborating with you on a document. Flag icons indicate where they're working, and their changes appear to collaborators in real time as they work.