It would be a pretty boring death game if you could just completely over-level all of the content.
The characters are in the high 80's level-wise at this point, maybe even 90, so I don't see it being that unbelievable that they could start maxing out skills.

The skill system is explained in the novel (vol 1, ch5 is basically the first half of the episode that just aired and explains how skills/levels relate). I don't know if stuff like that is considered a spoiler, so you can go read just that chapter if you're curious, or I can just tell you.