Corbyn had always been hostile to the EU, and his lacklustre campaigning for “Remain” contributed to the fact that fully one-third of Labour voters backed Brexit.
OK, let's take that a bit at a time. Corbyn has always been hostile to the pro-corporate element of the EU, but he's clearly not opposed to the general notion of continental co-operation or, indeed, to the strength of workers rights that come from the need for a single market. (Famously, he stated in an interview that he was about 7 out of 10 in favour of Europe. Given that the Labour result was basically 70% Remain, that suggests that he had a pretty good insight into the balance of his own party supporters.)
Secondly, he campaigned extremely vigorously for Remain, giving speeches all over the country on most days of the campaign. What he didn't do was appear on media platforms with his political rivals - should you think less of him for that? (After all, there's a good argument for saying that it was that decision which helped to cost Labour the whole of Scotland in the last General Election.)
Thirdly, if two-thirds of Labour voters in fact backed Remain, then that sounds like a pretty decent success to me in a campaign in which the final result was 50/50. It was the two-thirds of Tory voters that backed Leave that were surely the people who most needed to be reached, and they were unlikely to listen to Corbyn!
That doesn't mean that the general thrust of the piece isn't worth making, but it's this sort of repeating the same sort of half-truths that led half of the UK to largely believe what the Leave campaign were telling them, and for the people supporting Remain to be unable to articulate a coherent case.