Earlier quoted context omitted.
Even if the average returns and risk are identical for each security individually, you can minimise your portfolio risk by investing equally across many different industries. If a downturn were to occur, different securities in a single industry will be much more correlated, as compared to different securities across different industries.
Yes, that makes sense. But that's entirely unrelated to what I'm pointing out here. (Some of) the article is making the argument that investors should decide for themselves how over- or undervalued certain segments of the economy are. So on one hand you'd have a diverse market cap weighted index where you buy into stocks representing the proportional to their portion of the economy. On the other hand you might think…
However, if a passively-managed-fund with similar fees is claiming that you can lower your portfolio-risk by investing equally across many industries, that's an argument I find much more convincing. I had actually not considered this argument before, which is why I found the article interesting for bringing it up (perhaps tangentially).