My guess is that she is not this season's big bad.
From a narrative point of view, doing the reveal in episode six means episode seven has to have a twist up its sleeve.
In plot, she needs the readings from the vindicator, so maybe what has happened to Earth is as unexpected to her as to the Doctor.
I wouldn't be surprised if they became reluctant allies against a greater threat.
They’ve already said The Rani and Conrad are only two thirds of the big bad anyway. (We really all should move on from Buffy soon.)