Paul seems thrilled to have 'won' but given that it seems Tyson got paid more the longer the fight went on, he had every reason not to just beat the tar out of the upstart in the first round.
But also consider what Paul is bragging about here: going into a fight with a person easily old enough to be his father, and being proud that he won. Dear boy, when you have an advantage like a couple of decades of youth, you are reasonably expected to win by default. The fact you didn't take Tyson outright is demonstrative in itself.