I'm not guessing or interpreting. It's literally stated twice in one of the chapters. One time He Tians brother was talking with him about it (when He Tian got really mad at him afterwards) and the second time in the Christmas special when He Tian was talking to Lil Mo about how he moved countries. I read these parts today so I remember it well.
yes, but the last chapter before the confusing chapter(the one we are discussing) He Tian helping Lil Mo to pee. So after the peeing chapter there should have been a chapter in which the characters are having the He Tian is leaving conversation to make everything flow. But it went from He Tian helping Lil Mo to pee to everyone is sad without any explanation as to why they are sad. Those chapter you are talking about are in the past so they do not flow with the current chapters. But you are interpreting it that way because the story never explains why they are sad.

I just sitting here like ...... ┑( ̄Д  ̄)┍.... Huh?
Everyone is confused because it jumped to them being sad with no explanation
What happened??
and
Why are people saying He tian told Lil Mo he is leaving. That does not make any sense. That is too important to the story to just leave out. I hope author didn't do that because that would be so dumb and confusing.