In the 1880s, an archaeologist named Chwolson found headstones of a Nestorian Christian community near Issyk Kul.
So, a couple of things: 1) geography. Look how far Issyk Kul is from the Crimea!
2) Pollitzer's "pandemic spread" model might work if Issyk Kul were the only place where plague was present in the 1330s. But how do we know that? We only know ...
1) on Chwolson and the excavations new Issyk Kul: Uli Schamiloglu, “The Impact of the Black Death on the Golden Horde: Politics, Economy, Society, and Civilization,” Golden Horde Review 5:2 (2017), pp. 325-343.