How you gonna meet the floor boss when you JUST stepped on the floor, LOL. Unless this chimera is the basic pleb of this floor which means that is one hell of a spike in difficulty from floor four to five.
It isn't "having just stepped on the floor". Close enough for the sound to carry, but MC & co. had to go through some passages and encounters to actually get there.
Not many panels spent on that, but they are there...
This is also one of those dungeons which
is not conveniently "set up" for Adventurers, with one entrance and an exit after a "Boss Room", easily mappable, convenient glowing walls, etc...
This is a cavernous maze with multiple ways to get up and down levels, some of which may well have you enter in the vicinity of one of the Boss Rooms. And which may well not always be in the same place...
MC is right to be cautious and conservative... That maze is a literal death trap if you don't treat it with the respect it deserves.