What Kok-Chor Tan wants to show in his book is that liberalism doesn't have to be set against the existence of particular human cultures. But the arguments he uses are not exactly reassuring. In the part of the book I've just read, his argument is that:
a) Most human cultures can be redefined along liberal lines. The non-liberal elements are merely extraneous, or are oppressive impositions from above.
b) If there really do exist genuinely non-liberal cultures, then those cultures will have to be "let go".
He is giving cultures a choice: either you redefine yourself to be the same thing as liberalism or there is no place for you in our future global order. You could call this liberal imperialism or liberal supremacism.
In the following brief quote from Kok-Chor Tan's book, take note of his presumption that liberals rule the world and get to decide which cultures get the thumbs up and which the thumbs down. Note too that he treats individual autonomy as the decisive factor in deciding the worth of a human culture:
I do not deny that there could be, in principle at the very least, genuinely nonliberal cultures. When such hard cases do arise, we may be forced to make the difficult choice of letting a culture pass on and to try to accommodate its adherents in other ways. Remember, again, that our concern for culture stems ultimately from what it means for individual autonomy; so long as restrictions against individuals are a permanent feature (if this is indeed so) of a cultural way of life, we will have to concede that this culture will be one of those unavoidable losses of our social world.