He had a clear line of sight for the earlier part of the fight(when the Earth mech is getting eaten up).
There is no reason for him to assume that the mech is just 'passing' through objects.
this is really boring for me. if i watch through the scene repeatedly it becomes obvious that the interior of the armored vehicle being driven does not correspond to the exterior in almost any way.
the interpretation that idaho is able to quickly discern that a mech devours matter seems pretty preposterous given the level of motion involved, the limited viewing angle from inside the getaway vehicle, and the fact that debris is actually continuing to fly around, which would indicate that not everything in combat gets erased from the world.
for the sake of reference this is what it takes to get a clear view out of the windows from the back of the armored vehicle:
not the best vantage point for watching and assessing a battle when the car is literally being tossed around and kicked away.
where is idaho anyways (or half of the cast for that matter)? this is our view of the carrying area:
and this is our view of the front:
idaho magically shows up in that passenger seat the girl is leaving not long after:
let's take note of the opposite interior shot of the cockpit:
ignoring the compositional issues that exist because the height of the vehicle is being completely ignored here we run into another problem. the cockpit interior has a 360 degree viewing radius from the inside, but exterior shots show that this window only stretches around half of the vehicle...and offers no possibility for the passenger to look outside of the window:
summarize this however you want.