Besides, if his posts are in trashcan... shouldn't their number be shown as 0? Perhaps a mod can bring more light here...?
Technically no, the number will show exactly the number of posts they made.
Compare it with Investigation board, Serious discussion, Ivory Tower. If you make a post in Investigation board then below members can not see it but they see the post and activity count. On Serious discussion and Ivory Tower it's the same logic but that's for below Full member rank. So everyone can see the post and activity count but they can not see the posts that they are not allowed to access.