Something like this sounds a lot more plausible, and frankly more interesting, to me than time travel.
Making this manga a time travel plotline now would be like doing a stage magician hat trick, but pulling out a broken ipod instead of a rabbit.
There’s a whole subgenre of books based off this type of premise. Solo Leveling is NOT unique in that regard. It simply just handles the related concepts and world building in an interesting way.
I don't know why, but the chapter got taken down. Although after a quick search I found out that the group's name was Boredom Society.
Take that info for what it's worth.
Edit: Seems I was wrong about who the scanlator was. Thanks for the correction! @_das