I think it was a very human reaction because it must have still rankled him the way she dismissed him summarily in yesterday's episode and told him to push off when he was just about to melt and tell her something soft and sweet in response to her own apology. Remember this guy thinks saying sorry is akin to a gold medal (khushi's words😊) and here he had said a real heartfelt apology to her sister which would have cost a great deal of his pride to eat humble pie and admit his wrongdoing and it must have really bugged him all the more to have been caught at it by her especially after he had rather forcefully told Khushi that it was not her notion of stupid emotions that ran the world but money and power