If I remember correctly, there is an FAQ somewhere that says that they chose Swift because they were already familiar with it, and didn't know anything about Julia.
> picked Swift over Julia because Swift has a much larger community, is syntactically closer to Python, and because we were more familiar with its internal implementation details - which allowed us to implement a prototype much faster.
Personally I find it hard to believe that Swift has a larger community of people doing numerical computation, but maybe I'm out of touch.
The community of people using Swift for numerical computation essentially didn't exist prior to their announcement, and after their announcement it seems to only consist of the team developing the framework and Jeremey Howard.