Thanks for responding, Jingjo! I always had a feeling that the only thing that really made Robbins's ballet inauthentic was that he mixed in the other Asian dance traditions that Tuptim would either have no knowledge of or would be considered wrong for her to do. It's almost as if he did too much research, that if he had limited his movement vocabulary to Thai dance only, he would have had a more authentic piece. (But who knows? It might not have been as creative.)
My favorite moment in it is when the silk of the river is pulled tight and becomes ice. I don't think that is especially Thai--silk had been used for stage effects in Europe and America before that.
As far as the white makeup, I think that has to be considered part of the story, because if you accept that Tuptim even knew of Harriet Beecher Stowe's novel, you kind of have to accept that she also knew of the dozens of play adaptations, all of which would feature actors in blackface.
So it's part of the conceit and brilliance on the part of the creators of the musical to include the ballet in the show that places Asians in a ballet in whiteface/blackface reenacting Beecher Stowe's anti-slavery parable, as a way of one of the characters making a protest against slavery in Thailand and the creators of the show making a protest against racism.
The irony, of course, is that the yellowface casting and the whiteface/blackface ballet, over the past 60 years, have given the musical itself a tinge of unintended racism. Uncle Tom's Cabin itself has gone from seeming ANTI-slavery to seeming like something racist, so the very act of telling the story nowadays seems suspect.
I'm hoping this revival restores to the musical the progressive reputation that Rodgers & Hammerstein, Jerome Robbins and playwright John Van Druten (the original director) intended.