Quick run down from the novel version on what's happened:
Logar warn of demons setting things up. Taquila group recognize it as them preparing for eventual invasion by setting up tower to spread red mist.
Taquila side plan to use a magic stone that's basically spy camera to observe the demon's movement to know if/when the demons move out. However this risk exposing whoever go to put down the crystal to demons especially if they want to be close enough to observe things.
Tilly believe she's got an idea to accomplish this safely.
iirc she have the earth moving girl dig tunnels towards their target location, then the girl with the ground diving box go up to confirm their actual location/direction, after which they'll adjust direction as needed.
wait, this isn't MTL?
I mean..... even the most casual observer of the most rudimentary level would notice the errors here.
left with "wonder what they said at the end, but can gather what probably was"