They weren't cross-bred initially. (theres only a few hybrids) They were grafted (and most still are!) to the roots of American vines. The part of the plant the bears the fruit should be similar to the way it once was.
That is amazing, didn't know about the grafting. Seems crazy to create this kind of hybrids, especially considering how long this is done by humans. But I still think it will be hard to come by original vines from the roman age, as man tends to cross-breed or select all the time.