The idea is stated succinctly as Knoll's Law of Media Accuracy: "Everything you read in the newspapers is absolutely true except for the rare story of which you happen to have firsthand knowledge."

The earliest instance I found of Erwin Knoll's quote is from 1982[0]. I suspect either Michael Crichton or Gell-Mann had heard this quote, which in turn influenced their discussion.

[0] https://www.nytimes.com/1982/02/27/us/required-reading-smith...