Of course I have heard the argument. I think it's pretty rubbish. So there are two ways the argument could go: one is that homogeneity has some sort of independent effect on poverty (which—where does it happen? Why does it happen? What's the causal mechanism?) or that it politically makes implementing the policies that reduce poverty easier. With regard to the second, it's definitely true that the racialization of poverty in America has definitely helped opponents of good anti-poverty policy, but the solution to that is to combat racism so we can pass the good anti-poverty policies. With regard to the first, why would all the poverty reduction gained by "homogeneity" happen post tax-and-transfer (that is, welfare)? Why does it show up there and not, say, in market income, where Finland's poverty is higher than the US? How do we demonstrate that this poverty reduction is not due to the structure of their welfare state, but due to their homogeneity?

I think this argument also is bizarre from a historical perspective. About 100 years ago, Finland underwent a civil war, and in Sweden, people with more land literally got more votes in elections. When did the homogeneity kick in and start making a culturally unified, low-poverty society?

Ultimately the problem with this argument is also that there is just a much more plausible explanation: their economic institutions create very low poverty. You can see it empirically. It's very observable. Their welfare state designs are not a secret.