That wasn’t the Prime timeline, though, or it never really got the chance to be for very long - that we the timeline when Pike decided to write to the kids and avoid his fate. So while a butterfly effect may have ended up having Una still incarcerated, that wasn’t what we wound up with, not at the end of the episode, and not in “Balance of Terror”.
Having Una have temporal shivers from a timeline that no longer exists would be a neat idea, regardless.