It wasn't unbelievable if that was all they could do. After all, people with great skills could do the same thing, but Kirk told him that the two girls easily kicked over a car, which was incredible!
He couldn't believe it, but there was no need for Kirk to lie to him.
"Well, since you failed, then so be it," said the Chang family's eldest son. He decided not to work with Kirk any longer. Because Kirk failed, he wouldn't continue to help him.
"So be it? I can't accept that. Because of you, I've suffered a great loss. They weren't killed, but they were all disabled. I was paid back by them and I'm seriously injured now. I won't make a full recovery for several months. I'm talking to you with my last strength. You should hear that I'm very weak now and yet you did nothing! Do you think it's possible that I'll accept that?" Kirk was angry and said mockingly.
Hearing that, the Chang family's eldest son frowned, feeling unhappy about Kirk's response.