Except Malaysia, while awash with identity politics and with a history of racial strife, is also quite peaceful and prosperous.
Not leading the pack like Singapore, but way ahead of the US in terms of violence.
So... nonsense.
Authoritarian micromanagement might not be applicable to larger states (China tried to franchise the Singapore model without success, although not due to ethnic conflict), but there's plenty of multicultural nations at the very top of the list in terms of development, crime and political stability.
A fifth of Australians are born overseas and a quarter of New Zealanders, and they both rank up there along with the Scandinavian countries in all national comparison metrics.
Successful immigration policies are remarkably similar (as are the failures). Starting with not having the mass migration of an ethnic/racial underclass of unskilled labour.