When the CEO of the company appears on a national news program and states a date that is a pretty official announcement in my book. And when they have to make further announcements that it has been delayed that is further evidence that the announcement was official.

Waiting to have the terminal and all vendors fully functional makes more sense than opening it with problems. And unlike the BA/LHR T5 this change doesn't start a flow of actions that affect almost every other airline operating at the airport, so the 3 extra weeks doesn't really hurt anyone or anything.