The author made no logical sense. The main thesis is that “education costs have soared…but learning has stagnated.” In other words, the lack of productivity is about much inputs (high education costs) yielding too little outputs (low learning). Note that the author here is talking about causality. But if you read his blog or the report carefully, he actually picked and chose his talking points with no logical sense. When he wants to criticize high education costs, he talked about college education costs. When he wants to criticize poor learning, he talked about k-12 education outcomes. How could it be possible any logical causality exists from high college education costs (later in the time line) to k-12 education outcomes (earlier in the time line)? Give me a break.
A possible legit argument/hypothesis would be more like something as follows: (hypothetically) high college education costs in STEM yields little tech innovations, as a result, GDP suffered over the past 20-30 years.