While your evidence should be factored in, I am not giving up on my theory just yet. You see, I tried some Mandarin with her too, and she similarly had no reaction. I think if you're not expecting it, you may not realize this person who shouldn't be speaking your language is trying to speak your language.
That said, she DOES speak Mandarin. I heard her calling out to MMS or whoever in the halls. It's a weird accent - neither standard, nor how a Westerner speaking Mandarin sounds. I can say with 99% certainty she did not grow up in Beijing. Or any other Han-majority locale. 95% certainty she does not have a Han parent. And I still think she LOOKS like that "model" I linked to in my review.