According to Yayul Chojae, a Korean blogger, many of Southern East Asian populations including southern Chinese and Southeast Asians were recently generated.
So the higher genetic diversity among them which was taken as evidence for the origin of East Asians in the south resulted from admixture instead of greater time depth.
I think he is right at some level.
Very roughly
ANE R Q
Northern East Asian C2-F1067(northern branch of M217), N, Q
Northwest East Asian(this paper calls this Sino-Tibetan) ?
Southern East Asian O2-M122, C2-L1373(southern branch of M217), N-L729(southern branch)
Coastal East Asian O1, O2
From non-East Asians
Northern East Asians picked up R and possibly Q(not clear which is the direction)
Northwest East Asians, D and O(maybe)
Coastal Southeast Asians, C1 and maybe D for Jomon