It leaves us going out with no deal. It means there would be a mandate for no deal.
Anyway, he isn't going to allow a vote to remain because he'd lose millions of votes. He wants to pay lip service to the idea of a second referendum in order to keep his members quiet, but the last thing he wants is an actual referendum.
Besides, McCluskey and McDonnell have ruled out a remain vote and they run the Labour Party now, so everyone else can whistle for all the good it does.