If they were, why would past Yeon forcefully tied the red string to past kihoon?
Anyways, I just hope they successfully end the cursed (which is not, which is intended for a twist in the story) so they can happily live forever. And I believe the tasks is meant for them to be together.
I think their soulmates in their past life it's just what I think it is Dont come at me guys ╥﹏╥ ╥﹏╥