Was there another reference to her rank in that episode (or others) that I missed?
Not dialogue, or Disco in the strictest sense, but IIRC, her computer pad in the Q&A Short Trek did list her as "Lt. Commander Una."
"Q&A" was set sometime before "The Cage" and at least five years before SNW Season One if the 2259 date for SNW holds true. She could easily have gone up a grade in the intervening years.
In episode one, the Archer crew manifest also showed her as a lieutenant commander.
