Wasn’t the issue that Corbyn wanted a seat at the table during any trade talks involving the EU and other countries? Something which the EU just wouldn’t agree to.
He wanted the UK to be able to influence outside trade that would affect us I think. Barnier said he’d rather deal with Corbyn than the Tories as he had a better grip on reality.
I’m not sure how possible it was but Corbyn hasn’t had the chance to negotiate with them.