I don't think they did. This loop ended on them surviving and reaching the outside world. And Hyun saying "Happy B'day" was indeed confusing coz all I can suspect is that the final loop lasted for so long that it became one year ??? Or he might have been just wishing Hyungwon after remembering his birthday. Either way the babies escaped and can be together forever !
i think maybe its because all koreans turn a year older on new year right? so maybe hyunie wanted to replace youngwon’s shitty actual bday experience/memory (the first day of the apocalypse) with a better one which is that they successfully managed to escape both the apocalypse and the loop, at least thats how i interpreted it (=・ω・=)
Wait so did they just loop back to the beginning again at the end???