That said - we can't get it to work. Both are on 1906 (Monthly) and it doesn't work on older .docm or even newer ones. I have the .docm file in my OneDrive and shared, and open, but cannot co-author. Have you noticed the same?
Versions of OneDrive Sync Client are 19.103.0527.003 (colleague) and 19.123.0624.0002 (me).
Would be interested to see if anyone else has noticed the same...???