Since the climate has been cooling for the last 8,000 years (except for the last few decades), there have been multiple waves of immigration into Indochina by Chinese. Snow falls in Guangdong and people head south.
Of course, there was an initial entry of H. Sapiens into Thailand from Africa a long time ago. These initial settlers presumably ran into Denisovans, who exited to the south soon after.
Interestingly, "southern" Denisovans were significantly different from those in North Asia.
However, it is difficult to pick out the differences in DNA in modern humans because of admixture from Chinese in Thailand. More truly ancient Denisova DNA samples are needed.