Whether Mihawk's title is related to strength or not is completely irrelevant. Zoro's goal is to be the strongest fighter to ever exist with the sword, capable of besting anybody else with a sword. Roger so far seems to be the very epitome of that out of the present generations, thus, he will be getting surpassed by Zoro.
The fact that Roger not only has a named attacked with his word but is also displaying high haki usage skills with it, is pretty much signaling what Zoro has to aim at.
Which means, the guy who Luffy will be surpassing isn't Roger, but his own grandfather Garp. Garp being the only legitimate fighter type of that era, just like Luffy.