I can’t speak as to whether this was valid in the past, but it’s no longer the case that activity in /groups/TEAMNAME/activity for restricted categories appears to users who don’t have access.
You will only see activity in common or shared areas. So if two teams share one category and have additional categories locked down to their respective groups you will only see activity from the other team pertaining to the shared category.
If none of those areas are public then anonymous users and members outside those groups will see nothing.