All or most anyway, not sure about the chefs etc., SE staff have special mod status. Presumably, this is part of the special way that they are handled.
Basically, that's not a regular user but a Stack Exchange employee so different rules apply.
As for the cases where they have <100 rep and have not posted, they have probably downvoted. Each downvote on an answer costs you 1 rep.
Moreover, the negative reputation is not visible in reputation history of users to anybody but the user himself/herself.
