Sorry, inarguable was a bit strong. But the facts, as laid out above speak for themselves. Gus did NOT achieve what Mullery did.
Nor did Mullery achieve the style of football Gus did. I can understand why many see Mullery as the best ever, he was before my time so I can't say either way. In my lifetime, Gus is comfortably the best. I'm hopeful Oscar will surpass him though.