Context: Kaiba is talking with Dartz here, because Dartz just explained what he meant with the “darkness of the heart”. (Ep. 180)
This moment is pretty cool, because, if you see it from the Duel Monster anime-verse only, what Kaiba is saying here doesn’t make much sense. Kaiba himself maybe was “consumed by the darkness of the heart”, as shown in the first episode. But he wasn’t even as half as bad, like he was in the first manga chapters/Season 0.
And he keeps mentioning Mokuba, too. But Mokuba wasn’t even portrayed as an evil person, since the first episode.
So, Kaiba is referring here to his and Mokuba’s villain days and that they both were able to put their hearts back together after that. Especially Kaiba himself, because in the manga he really “carved out the light in his heart”, by putting his heart back together, throughout Duelist Kingdom.
The anime staff decided to make a reference to the first manga chapters/Season 0, and I think that’s just a cool, little detail.
“I’ll show you how well I can get over a defeat! I’ll lose to you as many times as I can and you’ll see just how over it I can be! Oh, I’ll be so over it you won’t even know what hit you, Yugi.”