There are Rogue-likes all over the place in this pack. I wouldn't assume there was no place underground where mobs could spawn.
100x100 isn't big enough even if there is no place underground. The despawn radius is supposedly 128.
It wouldn't be too hard to test. Just nerd pole up to 150ish then slab over 30 or so, build a small dark room, go back to the top of the nerd pole and see if something spawns quickly. I'm fairly confident it will.
I'm not far enough into the pack to need a dedicated trap. I have a moat with a trap/kill pit that gets me the zombie flesh/bones I need early game. I'm familiar with the layout of roguelikes so I know where to get endermen, blazes, and creepers so I probably won't need a dedicated trap now that I think of it.
Update: Just built this in a test world. Behavior is as expected. Built darkroom, moved > 24 blocks away for not even a minute, walk back, 4 mobs in the darkroom already.
Oh and of course this only works in SP or if you're the only player on the server since mobcap is shared.