That's exactly my point! It's not as simple as "it is open source, therefore it is gemeinnützig". It depends on the project.

Therefore it doesn't make sense to recognise "open source" as "volunteering". What makes sense is to consider "volunteering projects" as "volunteering projects", and the way one decides that is by looking at the project. Open source or not.

But I assume that's already how it works: to qualify as "volunteering", someone in charge has to look at your activity and confirm that it does, indeed, qualify.