It is unusual for a user’s content to be deleted in such a fashion but their user details be left in place. I think the regular methods of user deletion or anonymisation would normally account for this in practical terms.
That said, I think it is useful to know the effects of some outlier cases so we can form a picture, so thanks for mentioning this. 
If we get more reports this will help us look into the issue further, but for now I think I would advise admins/moderators in general to be mindful of the effects of manually pruning content to such a large degree, and to make users aware that it can have an affect on the veracity of their stats.