The way Kaiji reads the x vs. 2 situation is pretty simple. He realizes that if you cheat once, nothing stops a person, who's established that arena, to cheat even further to absolutely guarantee your win. The only thing that beats an A is a 2. So the fact that the Aces are established, and Kazuya is still confident is telling that he can still pull off a win despite the disadvantage in Aces. Kaiji doesn't know that Kazuya used 2 Aces per se, but Kaiji still has 1 additional Ace advantage that he can grind out to eventually snowball a lead.
So the only reason for Kazuya to be confident in that scenario is that Kazuya has a 2 that he thinks Kaiji wouldn't see coming. But Kaiji's already been through so many gambles where the opponent just cheats a step further. The biggest loss he's ever had was when somebody realized that Kaiji was about to cheat, so he countered it, and Kaiji didn't have a failsafe, thus losing. Kaiji plays on his guts, but once he sees Kazuya's confidence, that's actually when he confirms that he made the right move. Prior to that, he might not have known, and he would've also thrown away his potential Ace advantage, even though Kazuya had already used up 2.
He already knows the game is won, and he effectively has a life advantage, technically Kazuya has a red but won't necessarily need to use it as money is a small loss for him
