nooo its kinda like our mc is lwk emotionless so she doesnt understand why she was crying (it was bc she was too overwhelmed w the recent dares bc of mc and stuff) and the mc didnt understand why she cried and thought it was like did ur mom die or smth kinda like "why are r crying did something big happen" and its kinda referring to her mom dead ig but she didnt rlly feel sht IDK
yeah thats the point sarang doesn't understand why subin is crying, so she thought of any reason why she would be crying. the first thing sarang can come up with is when she cried remembering her mother, you can see that in chapters before that when sarang mom is mentioned she would cry, and probably when sarang mother died, she cried.
so she kinda thought "oh maybe she cries because she has a same problem (mother died) as me when i cry." we have to understand the way sarang thinks is actually very simple and direct

did ur mom die is CRAZYY