Because if you look at the symbolism in both life's the departure in the first snow and this closure at the moment is about the past (symbolism to their deaths) and Jooin wanting Cain to be in the present, a new proper beginning. Plus between the two, Jooin bluntly told yahwi that he wounded hum deeply and broken his trust, I don't see how Jooin will go back to him when his and Cain closure was more positive and more open to them reuniting.
Also it's not about Cain not being korean, it's about the author themselves, they are korean, the webtoon is a Korean webtoon. There's gonna be korean superstitions if they want to add it. And Cain says that it's snowing just like that day (when they died). Even in the flashback he said it's starting to snow. So I'm pretty sure it's their first snow.
Yahwi is done. So there's no worries because Jooin literally confirms in his monologues that the he can never get used to the pain, the wounds by yahwi cruelty are to deep and the trust is broken between. That alone confirms Jooin won't choose yahwi. Even Jooin says "I hope you meet someone else to tell the things you need". Plus the whole title change during Cain departure not yahwi. So it's obvious who's endgame.
Oh and another things is all of yahwi story has been told, his past and present. Cain, we only know of his past but not his present, which the author said will be shown in the story.
Just FYI there is a endgame, it's confirm by the writer. So looking at both closures I think Cain is still endgame because how different the closures are. Jooin left yahwi because of how badly he treated Jooin. While with Cain, Cain is the one that actually walked away, not necessarily Jooin but the reason Jooin wanted closures wasn't because Cain is bad but because Cain was stuck in the past and Jooin didn't like that, he wanted Cain to live in the present and not give his life up just for love especially if it's based in the past (which it no longer is but Jooin doesn't know that).
I'm honestly so excited to see what happens next. Because despite Cain and Jooin going separate way THEY HAD THEIR FIRST SNOW!! in Korea a first snow means two people will be together forever in a romantic aspect and it also means a new beginning, as their closure was more about the past.