The author was not presenting it as a serious hypothesis. They were showing how compelling it looks at first glance, and how deceptive that naive analysis is, because of the unfair advantage the chart gets (in the sense that it inherently looks better than its predictive value). They did this to knock down many other valuation-based measures, not to advocate for this one.
Reversion to what mean, is the author’s point. The mean for the twenty years between 1900-1920 is different from the mean for 1900-1940, is different from the mean for 1900-1960, etc. The mean is continually shifting. All data sets revert to their own mean, on average, by definition. The mere fact that a data set does so is tautological and thus provides no evidence — no confirmation of any scientific hypothesis.
Edit: see my other comment for the quote of what I consider the key passage in the article.
Reversion to what mean, is the author’s point. The mean for the twenty years between 1900-1920 is different from the mean for 1900-1940, is different from the mean for 1900-1960, etc. The mean is continually shifting. All data sets revert to their own mean, on average, by definition. The mere fact that a data set does so is tautological and thus provides no evidence — no confirmation of any scientific hypothesis.
Edit: see my other comment for the quote of what I consider the key passage in the article.