He initially thought that when the first bottle was halfway through, Hao Jian would already be unable to continue and was just holding on. So he mocked Hao Jian in his mind, but he never expected that Hao Jian could actually drink the second bottle. The key was that his drinking speed was even faster than his own as he continued.
Though drinking is about alcohol tolerance and how easily one gets drunk, if one's speed is significantly faster than the other's by the end, that also means losing a great deal of momentum.
So when Boss Zhu saw Hao Jian's speed suddenly increase, and the key was that he was now faster, already surpassing him, he immediately felt indignant and hastily sped up his drinking.
