> Talhelm said that one of the most striking findings was that counties on the north-south border – just across the Yangtze River from each other – exhibited the same north/south psychological characteristics as areas much more distantly separated north and south.

This from the original article seems to support your "invasion stopped at Yangtze" theory more than a "rice/wheat divide" theory. Rice is grown north of the Yangtze also and the border seems to be on the hills between provinces such as Hubei and Henan.