That's the first thoughts I had after seeing that passage. Doesn't it look like a future that hasn't happened yet more than another dimension?
Sayoung of this timeline doesn't seems to know about mc true identity yet (or at least it hasn't been show that he does 100 percent ) but he might know some bits an pieces unlike others. (Like fragmented memories of that time Which push him to keep Euijae near him despite not knowing fr why? )
That could be true too ... I just thought the case coz Sayoung has a weird obsession over the west sea rift where mc came out of ...
But did he got transfered to another world, like a multiverse?
Or did Sayoung have memories of that fragmented world ,like he got so broken after Euijae died and turned back time just to meet him?