Twitter confirmed that the attack used internal tools, and thinks the attacker used social engineering on employees: https://twitter.com/TwitterSupport/status/128359184496275046...
Which shows that Twitter probably doesn't properly employ 2FA and two-person-principle when dealing with high-profile accounts. Otherwise, social engineering would have been almost impossible.
I’m guessing someone re-used a hacked password and SMS 2FA is to blame. Maybe it’s not even that sophisticated.