Yusef is a key figure in the Holy City, and Li Ang has known about him for a long time.
However, his surname was never announced publicly, perhaps only the top brass within the Guild know this fact. Li Ang has had little contact with them, so he didn't know his surname is "Cha'ain."
This information is extremely important.
For ordinary people, the focus is on him being a descendant of Lichman Cha'ain, while for Li Ang, it can be said that his whole new journey since leaving The Soul Apex is closely connected with Zoe. And "Cha'ain" is mentioned in her father's notes, which are related to the secrets concerning her origins.
If he were just a Cha'ain encountered by the roadside, it wouldn't necessarily be related to Zoe's mysteriously-identified father.
But at the level of the Guild founder...
The couple is closely tied to the Star Souls hidden beneath the surface of the world.
Their standards are both very high, so there's a high probability they know each other.
