To show what the issue talks about in a nutshell lets look at my activity calendar when I am logged in...
Compare that to what everyone else sees and you get a very different picture...
As far as everyone else is concerned, I've barely had any activity this entire year. This could obviously be an issue. This happens if you have private repositories that you contribute to. GitLab only shows activity to users who are authorised to see the activity and there is no way to open it up beyond that.
To be fair the merge request for the changes for the above issue is complete. There is a catch though, it depends on another merge request that's still in progress. This other merge request has a lot of discussion, lots of changes, but it seems to have died in the past two months.
So what does this mean for being able to have the option to show private project activity? Well unfortunately until the merge request that everything depends on is resolved there will be no progress. Given it in itself has been going back and forth for a year also, I wouldn't be holding my breath while waiting for this feature!
-i