Given how huge this hack is, and how little the BTC reward is going to be, I'm tempting to think this is either: - a test of a new hacking system - a demonstration to a big client - a first shot to threat some entity - a diversion while they get the real loot And that the BTC messages are just a way to justify it so it looks like a simple scam. Such a hack is worth way, WAY more than the few BTC it could bring.
Hijacked the authentication cookies and injected into the app that skips validation for performance. Likely nobody got access to the accounts themselves but just allow them to tweet some jokes.