But my favourite answer to this question is this: So what?
Even if Harry was a foolish 17 year old who was wrong about that, and Voldy was the true master of the Elder Wand, Dumbledore’s plan has still succeeded. For Harry had, at that point, willingly sacrificed himself to defend all the others. This created for the defenders of Hogwarts the same magical protection Harry himself had received from his mother. And as such, Voldermort, free of Horcruxes and possible unable to create another, would be open to any attack from numerous people who he was powerless against, as we see that the body-bind on Neville and the silencing charms all failed to work for that precise reason. So even if Harry’s assumption about the wand was wrong, and he had no mekor, Voldermort was still almost assuredly defeated.