Quite simple. He’s in favour of the UK staying in the EU, even when a heroic wing of his own Labour party is anti-EU and wants out, because they correctly see that the EU is a corporate tyranny and threat to democracy and left-wing economics.
Democracy and the EU are incompatible, and more and more people in Europe agree.
Meanwhile, over at the European parliament, it’s left to the leader of the UK’s right-wing, populist Thatcherite party to complain that the EU is anti-democratic. Poor old Jeremy won’t be saying this anytime soon, so it seems.