First of all, I think Cao Cao is in his later years. That's true. Jia Xu belongs to a spirit of struggle in which he loses in the world competition, so I don't think it's just a question of who will rule the world. Jia Xu just wants to save his life, and he won't say what he can say. He just wants to spend his old age peacefully. After all, Jia Xu himself had already retired after years of intrigue, and naturally he would not be with Cao Cao. Maybe Jia Xucan really saw Sima Yi's ulterior motives, because Jia Xu was really smart, otherwise he wouldn't get Cao Cao's reuse, but for his own safety, he chose not to tell the truth and just retired.
Another possibility is that Jia Xu didn't see Sima Yi's intention at all, and what he said was just fabricated and muddled through. Although this possibility is almost impossible for the clever Jia Xu, there is a certain probability.
Whether it is true or not, Jia Xu is not telling the truth. Personally, I prefer that he wants to protect himself and think about his own safety.