I don't understand that as an exact and 100% time table. It is more a guideline. And I think that was a calculation with the existing Devs at that time.
If the team grows and more developers work on it, it can go faster of course.

On the other hand, it is not good to say we are finished definitely in Q2, if they don' t know how many Devs are available to do the work.
A lot of projects know the problems when they need to postpone predicted dates. This is also bad.