I'm not saying DM would necessarily have included a commitment to an EU refetendum, rather that I think it would have made it more likely as he would indulge in that most Blairite of tactics, triangulation. He would see it as a way of maintaining Labour voters who wanted out of the EU, with little actual cost because he wouldn't think he necessarily had to hold it.
Oh and BTW, facts that are prior to the hypothetical, such as Osbourne's promises to match Labour's spending, are what you use to base arguments about the hypothetical on.