I'm guessing one other reason he's staying is that he's probably still on Automattic's advisory board [1], and has significantly more influence on the company's direction than most other people who have taken the offer.
Even on the face of it this sounds too far fetched.
If he's in it for the mission, and not for the money, then surely you can't claim that there aren't companies with a more meaningful mission than blogging?
And if there are, could it be that he can't get hired there? He's been literally offered money to leave and he's struggling financially, but he doesn't.
This isn't the case for most people, but he is anything but most people.