I can't talk in too much detail, but in general, the deadline date was fixed through commercial contracts signed at a high enough level that engineering didn't have sight of them. The concept and commercial case was sound, but the implementation hadn't been worked out yet, when a date was set.
My strong preference would be for estimation to come first, of course, before a deadline is picked (and even then, only picked if it is really a necessary deadline), which is then based on reality, and also include some slack for unintended discoveries.
