Look at Japan's demographics, and opposition to foreign religions, languages, etc. They don't allow "the world" to recreate itself inside them, and the conflict and distrust (Bowling Alone, Robert Putnam) that that brings.
I'm sure being ethnically homogenous does smooth things over for Norway. But so does being small and galactically wealthy from extractive industry.
Like banning chewing gum. And the death penalty for bringing in drugs?
There was a lot of racial tension[2] in Singapore before Lee Kuan Yew straightened them out. Mandates on what languages to use for what ethnicities, with English being the main language, seems to have worked. As well as a racial quota system to prevent natural self-sorting into ethnic groups, and to enforce integration.
"To prevent conflicts and ethnic isolation (Singapore is the safest city in the world according to Gallup) ethnic quotas are mandatory.The distribution of public housing is carried out based on these quotas and it is impossible to find a block of apartments in which the percentage of Chinese, Indians and Malay people gives preference to one of the groups (with regard to the country’s population). And since the construction and ownership of the property are strictly regulated, given the lack of space on the island, it is extremely hard to escape the system. These quotas are also applied within the administration itself, in businesses and even in some activities such as leisure.To date, the system has been a success that has almost prevented any sign of segregation. However, the model depends on a system of surveillance, punishment and repression, which feels somewhat Orwellian, but without which, its implementation would not be feasible. This is, paradoxically, a society that voluntarily relinquishes certain freedoms related to the way in which citizens interact with one another, in order to guarantee their coexistence."[3]
[1]https://www.asianews.it/news-en/Ethnic-and-religious-violenc...
[2]https://en.wikipedia.org/wiki/1964_race_riots_in_Singapore
[3]https://tomorrow.city/a/forced-integration-plan-singapore
If you didn't, homogeneity really isn't the best word to convey the idea that social trust comes from long-standing, slow moving cultural traditions.
Even then there are counter examples but it could be one of those 'necessary but not sufficient' situations
I'd argue that Mumbai/India or Lagos/Nigeria are overwhelmingly racially homogeneous. So there must be some additional factor beyond racial homogeneity.
Europeans also had a lot more tribal-based racism in the past; southern vs. northern Italians, Scottish vs. Welsh, even sub-clans inside both of those nations, etc.