Come now, attempting to edit the subject so it agrees with your perspective...
Mike was asked a question, and answered. He was asked if he agreed with the notion, but how many Fighters could bio-mechanically explain how they hit so hard? I'm pretty sure he's not a physiologist, nor has done much reading on the subject particularly.
And you're making a very broad statement. "No one is completely limited." Of course. However, to overcome certain aspects of genetics is near impossibility. If it weren't, I'd love to be taller. I can take growth hormone, or attempt to stretch my body and may have a modicum of measurable success, but not to the degree that I'd get drafted by the NBA.
People can strengthen, when you learn proper technique, you can hit harder, sure. But I'm positive Smitty and Mike were referring to the single-punch killers when referencing "punching power." That's what's typically meant when two guys who made their living throwing punches refer to "power."