Narrative reasoning doesn't work in this case, because we don't know how the story will evolve. We are not Oda! What if he plans a big twist: Luffy defeats Burgess on Dressrosa, Blackbeard is furious and fights Luffy. We only assume Blackbeard as a final villain but he could be fought in the next arc. We can only rely on what we've seen, we saw JB win the A Block, but you can notice that unlike the B, C and D Block, Oda didn't give the names of the strongest fighters of this block. This is an indication that Burgess was the only fighter with a name, his easy victory can't be a proof of his strength. The last time Burgess was in a real battle, in Marineford, he was mostly useless and only helped to kill a dying Whitebeard. Assuming Burgess is equal to Marco is crazy.