I'm not 100% sure that you read the whole article. There's absolutely a reason given why per student expense wasn't considered: "intentions to raise performance are not the same as raising performance." That's a good argument for expenditures not to be included regardless of the impact it may have. Why use a questionable proxy when you could find better ways of actually measuring performance?

The "high-tax, high-spending progressive utopias" remark was mirroring the Paul Krugman op-ed that was mentioned in the introduction to the article.