The author didn't write which translation this was, but from the sentence length, syntax, and Google, this appears to be Benjamin Jowett's (1817 – 1893). I don't understand his 19th century English translation.
What exactly did behavioral research disprove? What did Plato bungle? Please see the embold phrase below.
WHAT CHOICES ARE WISE?
This question has been discussed for millennia. Many proposed answers share a common theme, eloquently expressed by Plato in The Protagoras:
What measure is there of the relations of pleasure to pain other than excess and defect, which means that they become greater and smaller, and more and fewer, and differ in degree? For if any one says: “Yes, Socrates, but immediate pleasure differs widely from future pleasure and pain”—to that I should reply: And do they differ in anything but pleasure and pain? There can be no other measure of them. And do you, like a skillful weigher, put into the balance the pleasures and the pains, and their nearness and distance, and weigh them, and then say which outweighs the other.
Behavioral research has shown that Plato was wrong [Emphasis mine]: The tradeoffs among different pleasures and pains depend heavily on context. “Skillful weighing” has to take account of how weights vary with context. A more flexible definition of rationality must be sought, one that acknowledges a variety of ways of solving decision problems.
Much behavioral research suggests that decision makers are often unwise. One can scarcely disagree! Yet arguments about what is wise must be delicate, because behavioral research also suggests that standards of wisdom need to be set with great care.
Paul Slovic, The Irrational Economist (2010), p 65.