I currently have no direct reference for what the OsmAnd developers have in mind, but OsmAnd is not alone with this, and the developer of a different App gave some insight, see https://community.e.foundation/t/list-apps-that-work-or-do-not-work-with-microg/21151/180?u=anotherelk
Edit: At FairEmail - Status it says at the bottom of the page in the old, obsolete FAQ:
“Would it be an option to distribute the app on GitHub, F-Droid, etc only?
About 99 % of the people using the app download it via the Play Store.”