I don't see why you think the premise is that people are purely motivated by self interest. The premise and conclusion is spelled out quite plainly. Matt Ygles

I don't see why you think the premise is that people are purely motivated by self interest. The premise and conclusion is spelled out quite plainly. Matt Yglesias is claiming that Americans underestimate the wealth of the poor and overestimate the wealth of the poor. He concludes means they underestimate the needs of the poor. I think you are making the assumption that what is being promoted is anti-wealthy as opposed to pro-poor. I think it's probably safe to assume the authors consider themselves in the latter.