Bloody Disgusting later said they confirmed Phoenix to be the working title, not the official title.
But, as the site acknowledged, it could’ve also been the working title — the way Project Angel was Terminator Salvation’s working title, and Vista was Terminator Genisys’ working title.
The Terminator Fans uncovered some posts from one of the supporting cast members, and the actor referred to the film as Terminator 6: Phoenix.
However, it sounds like the working title used during filming was Terminator 6: Phoenix.