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It's not true is it? For the following reasons;
Corbyn is credible because he is principled and consistant. He was not an effective Remain campaigner because he did not support it unquestionably. He's not a fan of the EU. Whilst that kind of honesty is unusual it's not a failure unless (as has become normal in politics) you only measure short-term outcome.
The Remain campaign was shit anyway. Almost nobody had their vote swayed by a Remainer political leader.
Corbyn's approval rating with the public was litterally lower than a random, unheard-of person you could drag off the street. He'd have convinced nobody. The campaign knew this and didn't want him around.
It has been observed that if only Corbyn had devoted the same energy and enthusiasm that he has shown during this campaign to the referendum last year then there is a fair chance we would be in a different place.