It's very simple really:
The only way to fight and beat a boxer with WC is the same way you fight and beat anyone else with WC -
You expose your WC to said fighting system, learn the puzzle it presents, learn how to solve it and voila, you have effective WC vs whatever system it has been exposed to.
The boxer had all the advantages:
Height, reach, power, skill level and confidence.
He also had the full contact experience as we can see that when he does get hit, it doesn't phase him, as opposed to the WC guy that gets his "structure" compromised with virtually every hit.
Psalms 144:1
Praise be my Lord my Rock,
He trains my hands for war, my fingers for battle !