A few days ago i made a post about these Credits because I also wondered about.
Helpful Links which can explain what these credits are, can be seen inside the GDevelop documentation:
https://wiki.gdevelop.io/gdevelop5/interface/profile/credits/
https://wiki.gdevelop.io/gdevelop5/interface/games-dashboard/marketing/
They are writing this about Credits:
" When credits are acquired, or won (by participating to a game jam, unlocking an achievement or other specific actions in the community), they are tied to your GDevelop account."
But: It seems that achievements inside the app or in the forum does not increse the won Credits count, because I personally got no credits yet for achievements.
Maybe they mean a specific achievement which is very hard to optain?