ProtonVPN codebase source delay given new releases on Google play

Given that there hasn’t been any commit pushed to ProtonVPN github with 5.18.75.0 release last week, there is a new version of it for update in Google play. Does this mean we aren’t quite installing “open-source” apps? (at the time of release i mean). when people want to verify their app source code, what are they doing if source code is uploaded after people have installed the app already?