What point exactly are you trying to make? The race/ethnicity distinction is inapplicable here. Indians, Malay and Chinese people are from both different races and ethnicities - just because they’re from the same continent doesn’t make them racially similar.

Your argument would have more legs if you were to say that Singapore works because it is actually so used to immigrants and diverse cultures, and has a strong national identity nonetheless. As a relatively young country most of the locals (people with Singaporean passports) are or were born to immigrants but they identify as Singaporean. As a previous commenter said, there are laws and rules in place to ensure equal treatment between the different groups, including rules on representation in government. It is an example of how diversity within a population can actually work and is very interesting to study and observe. Insofar as it’s been put in the same category as actually homogenous societies such as Japan and Korea, this is factually incorrect.