Nie Li is being quite crafty here. It's also stylish he isn't trying to win over the old man directly. You'd think he would be better received if Ye Zong knew how it's basically all thanks to Nie Li that Ziyun is cultivating so fast, but by not revealing that, Nie Li can prove his character better. Boasting about his achievements would be petty and uncouth. It's far better if Ye Zong finds those things out on his own.