So I'm betting that the world ended due to some form of nuclear catastrophe. Iirc, radioactive isotopes are unstable and from what the big "nuko" is said it seems that those creatures feed off of nuclear waste or something similar
i wonder if humans were the ones who created these creatures. because its seems possible they created them to combat nuclear waste. im pretty sure they were also used to eat trash like plastics which is really convenient.
When the one says "Last time... We fell asleep..." it makes me think that they are some sort of guardian that was meant to prevent whatever catastrophe happened but were unable to due to the fact that they fell (were forced?) asleep.