We're assuming they'll be making the "average amount" of mistakes. We're assuming they are both human and will make human mistakes. If both parties were to be playing perfectly, they'd just constantly dodge, roll and out-space each other, taking almost no damage.
We're assuming that at the highest level of play, both sides will make mistakes, but not inordinate, huge and stupid ones. We're assuming that they'll also possess the knowledge, training and skill to minimize their mistakes as much as possible.
How often do you see someone at the highest level of play die because they Up B:ing into the side of Final Destination? Not very often. How often do they die from aiming Sheik's or Zelda's Up B:s wrong? Not often. How often do they miss that chaingrab? Not often.
We're assuming they'll be making as many mistakes (and of the same magnitude) as people usually do in your average match and the highest level of play. We're assuming that since they're on roughly the same level, both sides will also commit the same amount (and magnitude) of mistakes.
So we are not going to assume that D3 is going to keep screwing up the infinite, SD and just run into DK's moves while DK does everything perfectly. In a game where both sides are making the same amount of mistakes and playing at the highest level, what would the outcome be?
DK would lose. Every single time. It is an unwinnable match-up, an auto-win/auto-lose. Deal with it.
