Hyunmook and his other members had been there for 80 years༎‿༎ (as far as I remember)

dows time flow differently in the abyss???

Yep! It actually goes the same with other manhwas that has this kind of apocalypse

For example in the real world only 3 yrs has passed but in the abyss its goes 10x or more than

Spoiler!!

Yes, Yohan had already been gone for two weeks in the real world but in the abyss he spend 1 entire year there.