Think of it like this. Hakumen was brought towards the current time. Since he was in the boundary it means he exists in the current timeline. The time loop was stopped and Jin didn't go back in time to become Hakumen again, BUT a past Jin did become Hakumen and is alive right now. So in reality, since they are from two different instances of existence, they are allowed to exist among each other. This Jin now no longer has the future of becoming Hakumen and is a separate Jin from the one fated to be Hakumen.
Not sure if that was clear enough but yeah, 2 Jins from different timelines.