Nice knowing that the SEC investigation backs up his story.
Every security team should be proactively trying to eliminate security vulnerabilities. But they will happen nonetheless, and the mark of truly secure organizations is in the way they respond to such vulnerabilities.
At the CISO level this becomes even more important: a CISO is not only responsible for setting the organizational mandates for security. They are also responsible for being the public face which responds to notable security events.
The CISO is fighting against the mindset of all other managers in the company who just want to "deliver" the features to make their numbers for the quarter (so they can get a bonus or be promoted), and the developers who are required to "cut corners" both in terms of quality and security in order to ship.
Both Yahoo and Facebook have a history of insecurity that dates back far before the arrival of Alex Stamos, who as others have said, is the "fall guy" when the leaders of the organisation prioritise short-term profits/growth over the security of user's data.
-- Winston Churchill