She's already put all the pieces together. She's willfully denying what she's already learned. Maomao doesn't know if Jinshi knows or not. She doesn't know who knows at all. But she knows if she provides any hints that she knows the truth, there's a high likelihood it is being kept secret on purpose, and she will be killed to keep it that way.
Only we the audience know that Jinshi has no solid proof or any firm intention of pursuing them either. Only suspicions he lacks the proof of, and he's certainly not going to ask Ah Duo outright about it. We don't really know if he has any intention of acting on it either. We know that Ah Duo knows, Maomao's adoptive father possibly knows, that Maomao figured it out, and that's the extent. Everyone else we know of who knew is dead. We don't know if the Emperor knows, if the Empress Dowager knows, or anyone else is holding it as a secret.
That's worse than position Maomao is in. She has no idea who is would be than happy to kill her to ensure that knowledge doesn't spread. She's exactly the type to push it down and pretend she never learned it, because that's the society she lives in.
Of course this is all intentional by the author. It creates tension. There is a looming threat now that someone is going to find out Maomao knows a very grave secret. Or that Maomao slips up. Or that someone asks her and she fails to hide it in her expression (just like Jinshi figured out she can read with a trap set just for her).