Elder Lu naturally realized that Zhang Yi was stronger than before, but she thought this was normal because he had made many advances in his cultivation and did not know about the essence of the phoenix flame he had.
In other missions he also wanted to further help elder Lu who was learning a lot by seeing Zhang Yi use his supreme understanding of the laws of space, so he decided to reveal that he also had a supreme understanding of the laws of water.
That's because elder Lu also had an advanced understanding of water laws at level 3, but he had heard that she had temporarily given up on improving her understanding of that law until she came to the supreme understanding of the laws of space.
But with Zhang Yi showing her supreme understanding of the laws of water, she would learn something for sure, especially since it was easier to understand the laws of water than the laws of space.