The author asks the question, can we apply something like the Efficient Market Hypothesis to other fields, like philosophy? Recall that in economics, the EMH has strong and weak forms, and his still hotly debated over which (if any) interpretation is correct. In the strong form, the market price of a good (typically a stock) reflects all possible information, whereas in the weak form temporary distortions are possible due to frictions.
By extension, if this concept is applied to the history of thought, you might come up with something like, for any good argument you hear, someone has already developed a good counterargument. In a strong form longstanding philosophical questions still being debated today are genuine stalemates. History has eliminated the bad responses to questions of free will, the nature of the mind and matter, the existence of God, etc., and the arguments that remain are the ‘good’ ones. The weak version might be some variation of ‘there’s a literature on everything.’
It’s a fun little read and something to think about, especially as I’m reading a couple of pieces right now on the history of ideas and whether they affect history.