Again the explanation

After Hunter World Seonwoe's sacrifice, Jehee regressed to the past again. It was during this regression that Seonwoe finally realized the truth, he had never been a person from Earth. He had always belonged to the Hunter World.

Using the watch in his possession from Jehee , the regression returned to the point in time when the first dimensional switch occurred.

At that moment, you can see still, we are back to the chapter 1 where Hunter World Seonwoe believed he had been transmigrated into a game which in fact, however, he wasn't entering a game at all, he was unknowingly reliving and correcting the events surrounding the very first dimensional switch at the age of 9.

This version makes the timeline much clearer,

Jehee regresses → discovers Seonwoe's true origin → uses the watch → returns to the first switch → Seonwoe mistakenly believes he's inside a game, when he's actually fixing the original dimensional swap.