Maybe Nat is trying to counteract any PR damage to GitHub, or maybe he just wants to see how much can skyrocket his account's karma. If I was in his situation I will be happy doing it for both reasons.
If Nat doesn't gain developer and corporate traction, Microsoft replaces him in a smash move of hiring a well-liked CEO. However, if he is lauded by developers, then he stays as CEO.
(and yes, I destroyed the account I did an AMA on a while back as well.)
Unrelated, but I really like your blog[1]! It's one of the few I read.
1: https://blog.codinghorror.com/ (for the people that don't know it).
He may even be interested in taking the pulse of some of the community and learning from interesting questions.
I know I would.