I suppose the guardian filters group members’ activity based on the permissions of the logged-in user. So, for example, if members of the “camille” group, and the person observing, are also part of other groups that have access to other categories, there’s no reason for their posts in those other categories to be filtered. 
Should they be?
In my opinion, what’s really being sought here isn’t a group’s activity, but activity within a category, which is close enough if only one group can read/post in that category. Personally, I tend not to let group members access the member list (the group is visible only to the owner) and instead direct everyone to the categories.