It makes sense now to me because not only can they tolerate extreme cold but also extreme heat. There are a few cherry relatives that are low chill, like Prunus ilicifolia, Prunus salicifolia, and Prunus lyonii as they are from subtropical areas like California and Central America. I don't know of their quality first hand but I do know their fruits seem to be more similar to the common cherry in size and flavor. Seeds of these cherries are sold on TradewindsFruits.com when they are in season, if you would be interested. They also sell wild black cherries, chokecherries, mahlab, and your Nanking cherries.