All achievements do not signify omniscience.
Su Mo only knows what he's experienced from a "player's perspective."
He doesn't know the content that didn't appear in the main game "Dawn" or was undocumented in the official setting book.
For example, if you asked Su Mo when a certain quest can be accepted, where to pick it up, and how to complete it, he could recite it fluently.
But if you grabbed a random NPC on the street and asked Su Mo who this person is, what they do, and what their background is, he certainly wouldn't know.
For Su Mo, Dongdong fell into the second category.
In the "Dawn" game, Dongdong was just an NPC who could assist in combat, able to help fight after being successfully recruited.
But the developers didn't assign her any side quests, and the setting book didn't mention her background either.
Therefore, Su Mo knew nothing about Dongdong's past.
