The subpoena wasn't just for the public contents of tweets; it was also for any and all user information for the account, including email address and an anything else twitter might have collected, including IP addresses, physical location, browser type, etc.

In addition, the court ruled (yet again) that the defendant had zero standing to challenge (or even know about) the subpoena since it was directed at Twitter not him. Which is kind of terrifying given the push to cloud storage for everything.