I'm going to call the original Wui Wu from the 1st timeline W1. Wui Wu from the 2nd timeline who was just swing Jaejin (MC) dying is going to be W2.
W1 timetraveled MC into the 2nd timeline but couldn't go with him because his body would not be able to handle the power. Back when they 1st met, W1 and W2 were close to the limit. Any extra power would make him go beserk.
Because W1 couldn't go with MC to the 2nd timeline, W1 stayed at that "dreams space", I'm calling It the limbo, watching MC and W2 interact. W1 is as jallous of W2 as W2 is of W1 every time MC told him about W1. Well, maybe W1 is even more jallous because he was watching.
Anyway, W1 erased MC's memories from his family and childhood, as we know from several chapters ago. W1 is pissed W2 told MC about it because W1 wanted to be able to tell him himself. W1 thinks MC hates him because of it. He thinks MC might have forgiven him if W1 was the one telling him.
Now W1 feels like he's an extra in all this situation because MC seems to be a lot happier with W2 than he ever was with W1.
And MC is kinda just finding this all out now because his dying memories from the 1st timeline weren't complete. Up to now MC thought that space was a dream or something like it.

I kinda understand nothing of chapter 74 can someone explain it to me like I’m 5