For some reason Nuland went off the script that Rubio was expecting, endangering the Demonicrat/RINOcrat/DOD image. When, in desparation, Rubio tried to steer Nuland back on track that the notion of bioweapons labs in Ukraine was merely a Russion propaganda ploy, Nuland stated that "we are working with the Ukrainians on how they can prevent any of those research materials from falling into the hands of Russian forces should they approach."
It's pretty obvious that the research materials of concern in those bio labs aren't part of a cure for acne.
Thus the gaslighting order has gone out to the members of the fifth column enemedia to accuse anyone asking further questions about DOD/IC involvement in Ukrainian bio labs as being Russian dupes.
I agree with your evaluation. Rubio was surprised by the answer and was clearly trying to avoid what should have been follow-up questions.