British remainers were/are often leaning left but didn't savvy the EU anti-democratic nature. This article makes it perfectly clear as to why Jeremy Corbyn wasn't a remainer, per se.
The EU is a technocracy, not a democracy, designed around as much laissez-faire capitalism as it could get away with.
The EU isn't going to reform in time such that Jeremy Corbyn will be able to openly state that the UK should stay in the EU. He saw what's happened to Greece. He sees what's happening to Italy. There's simply no hope for the EU because the Germans just won't open their eyes to see the future as it could be were the EU to become truly democratic.
Does it mean that fascists will rise up to overthrow austerity along nationalist, racist lines? It doesn't have to mean that if various nations leaning left get with the real democratic program and stop cowering under the capitalist lash but rather take all the whips away and burn them.