They needed to reset credentials so this could've never been a stealth attack. By making it public, any later leak of DMs is much more likely to be accepted as authentic. Without that, most people would've doubted the authenticity of leaked material.
Precisely. And who's to say which leaked DMs are real and which ones are faked? If you're interested in this kind of stuff, I recommend the book Active Measures.
Perhaps it is a form of proof that they actually have access to the accounts and thus the DMs. Just posting claimed DMs that can be deleted and denied has a lower probability of being believed.
