I’m not convinced that these problems would be addressed by a smaller population. On average, people today are better educated, more peaceful, and better nourished than they were 100 or 500 years ago. I think this is mainly due to technological progress rather than population size.

I didn't read it as saying smaller populations would fix this necessarily, but that we should focus on fixing those problems before we worry too much about falling birth rates outside of countries like Japan and Korea that have such elderly populations. Even if letting the population drop doesn't directly cause the prevalence of these problems to drop as well, I think trying to force the matter of growing the population before addressing these problems would likely only aggravate them. We can't necessarily count on technological progress to keep up with infinite population grow, so we'll continue to outpace our ability to find adequate solutions with our ability to generate massive problems for ourselves.