Why would anyone, who is not an ardent Jeremy fan, believe he would remain in the EU.? Jeremy's after his own kind of 'no deal' with the EU.
Corbyn's whole political life has been in a anti-EU stance, even before most of the ERG were! Jeremy is a man who has been, in political terms 'baying at the moon' all his life. By a stupendous mistake the Labour party adopted him as its leader, then allowed his 'shock troops' to as it were storm the bastille and make sure he got elected again. Given his Marxist beliefs it would be reasonable to assume that once he obtained power, by whatever means, he would be difficult to shift. This would be nothing new as Jeremy holds fast to his views and in fairness always has, that has been one of his attractions to many of his disciples; however let no one be fooled, Jeremy wants out from the EU as much as any euro-sceptic Tory, if he can use Brexit to get his hands on the leavers of power, then all his revolutionary dreams will come true. Its something of a minor miracle he has got so far... but then they said that about Trump!