I don't think devcat uses the term end game. I think they refer to things in general terms like high level or low level. But the quest for the free passes is given to 7k+ players so it's towards that end of the spectrum for sure.
they actually did use both of those terms; end-game and high level players.
so what will perhaps be one of the best items you can get from the new dungeons will come from the same chests that some people used to think opening would decrease your end reward huh? well, i guess this rereconfirms that that rumor is false
it's always been a superstition.
but personally, i believe it decreases clear reward chest's grade.
but then again, it's just a superstition.