I’ve yet to see someone define what there supposedly being an historical Jesus means, though, anymore than Abraham Lincoln, Vampire Hunter being based on a real person does. It doesn’t tell us a darn thing, I’d say, about the veracity of any of the stories in the bible. Nor do real historical events in the bible mean there was a historical Jesus any more than Jack Reacher is proven true because there was a cold war.
So, I dunno about a historical cross. But I’m sure you cold build more than a few houses with all the supposed fragments of the true cross put together as particle board 
