Simple - he is going to tell you Walcott is a better and more skillful fighter than Tua and that Tua would not land the punches Walcott did.
Walcott was a technician, who specialised in landing punches in an unanticipated manner. Tua was a power puncher, who was relatively predictable in how he landed punches. He would land on Marciano, but Marciano would anticipate most of them. Either way, a segment of film like this can be misleading. I could post footage of Tua getting roughed up by Chris Byrd, and say what if that was Walcott, let alone Marcianio.
I don't think Tua was that predictable if he was ,I doubt he would have a 73% kayo record .But you can't post footage of Tua being floored by a near 40 years old 188lbs man,nor by a 37 years old 196.5lbs one. What if that was Tua landing those shots?:think
What makes you think that Tua needed an improved skill set to land the kind of punches Walcott floored Marciano with? Did you watch the clip? There was nothing sophisticated or artful about it. Walcott just walked Marciano down like he was a bum (just as he had insisted he would do before the fight, according to the commentator). It was no more difficult than the punches Tua landed to take out Ruiz or Moorer, really.
Tua is an the GOAT on paper but when he steps in the ring with top opposition he ALWAYS comes up short. 49-0 deal with it.