There was a genetic study in Japan that showed that Kishu was one of the parents of Satsuma. Kishu probably has less cold hardiness than Satsuma, however, since it was likely the other parent (kunenbo, also believed to be the parent of Keraji and Bloomsweet) that conferred cold hardiness. My guess is that the cold hardiness of Kishu isn't anything special compared to other mandarin varieties, but mandarins as a group do tend to be fairly cold hardy, certainly more so than oranges.