Was there a leak or something that the Siamese would be one of the cultures in Humankind? I went to their Twitter page thinking they were the most recent one announced, but it was the Russians.
I'd actually forgotten about the "only Southeast Asian country to never be colonized" thing in my earlier reply, because it tends to be blown out of proportion (Siam had to give up a lot of territory to the British and French to maintain independence), and also because my country has been repeatedly colonized, in contrast.

But yes, that is a factor making Siam/Thailand a good candidate for inclusion.