My impression was that, that was a grown up X with his kids.
Mio still takes notice of Dongtae, instinctively goes to protect him, and holds his hands. It seems it could potentially go down that route.
Previously, Dongtae told X that where he would get beat up daily wasn't hard to find, but no one would rescue him no matter how much he struggled or screamed. Although he didn't use the dices power for himself, he was able to alter things incrementally by being selfless to others... Jieun was saved from her reality, and Mio was saved from death... And as a result they're here to protect and be with Dongtae.