But China is also pretty smart, and as far as I know it doesn't have a North-South IQ gradient: http://akarlin.com/2012/08/analysis-of-chinas-pisa-2009-results/

That doesn't show the absence of a gradient, because they're reporting, if I'm understanding the description right, a PISA-aggregate of 12 provinces; the only other scores are places you'd expect to be outliers and unaffected by any evolution (Shanghai, Hong Kong, etc). There is a map, but:

all perform well above average according to stats from a Chinese online IQ testing website.

Yeah... Plus, note the striking East-West gradient. So this map is serving more as a measure of economic development and Internet access than a random sample demonstrating lack of gradient.

According to Wikipedia, the genetics of the Han Chinese is... complicated.

I'm not surprised. The Han have been expanding relentlessly for a long time.