As it turns out, my suspicion was correct: the quote mentioned was from 1977. Unfortunately, that paper's brief mention of keyboard layouts is then based entirely on "Human factors in international keyboard arrangement", which was published in 1975 and does not seem to be available online.

The Fable authors mention fatigue early in the paper, but fail to provide any evidence on this point. Their section on ergonomics focuses exclusively on the question of whether Dvorak is faster.