The Daming Dynasty, although restrictive towards women, was not as stringent as the previous dynasty. It is generally frowned upon for women from the backyard to be present while men discuss affairs, but both Han Yu and Deng Tong didn't mind, and they were just chatting casually, so Su Wenyue didn't excuse herself.
People of different statuses discuss different topics. Han Yu is an ambitious man, or perhaps it's better to say he has ambitions, which is why he became acquainted with Deng Tong. While with Deng Tong, they predominately discussed matters of the imperial court and the local government. Su Wenyue actually wasn't very interested in these topics, but due to her experience from her previous life, she was not unfamiliar with them and could understand.