Well, Sino–Tibetan is one of the only places I would expect to see pharyngealisation in the first place. I can easily imagine a scenario in which vowels had a u

Well, Sino–Tibetan is one of the only places I would expect to see pharyngealisation in the first place. I can easily imagine a scenario in which vowels had a uvularisation or pharyngealisation contrast, which then was transphonologised onto the previous consonant. But I don’t know too much about Chinese languages, so until another explanation comes along I can only accept Baxter and Sagart’s conclusion.