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She's entirely wrong and lying, he literally and demonstrably campaigned for remain. There are arguments to say he's not personally a "remainer" and maybe his campaigning wasn't that successful/productive. He certainly did a lot of it though, maybe his less than binary black/white, leave/remain argue a point but consider the nuances seems against us because you're not with us now that things are even more polarised than they were then.
She's not really wrong though. He is at best ambivalent about Remain.
The Labour Party staying neutral in a 2nd referendum would be pretty shabby.