My interpretation would be libel or defamation does "hurt others" and would go against Mill's thinking. From wiki, "He argued that even if an opinion is false, the truth can be better understood by refuting the error." However, I think the opinion is something not harmful to others, say "The Sun is blue." I know he says libel is absurd, but I have a feeling, may be wrong, that libel at that time was different. Saying "i don't like this" could be libel. In fact, in certain countries you cannot say that still today. "I don't like this soup" could land you in jail.
Real Trader has no right to name names and say they cannot do this or that, unless he is extremely confident in his position.