Actually, protection from bombardment (which is distinct from bombing) is why WWII soliders started digging foxholes in the first place. The earthen walls are quite good at blocking shells, and since the ice spears don't explode they'd be even more effective at blocking anything that wasn't just falling straight down, which the first attack wasn't since they took shelter behind the barrow (and the Sentinels' comments on wind speed suggest they expected the spears,if any, to come from the cloud).

(Also, it gives them something to do in the face of panic which is useful in its own right.)

And even the much shallower shell scrapes they actually dig are better then nothing.

So if it was explained it'd be a genuinely clever moment, because it would show off Axis' ability to think on his feet and access novel situations. Since it wasn't it's a really bad moment because he's had information he shouldn't know beamed into his head.

And yeah, the song would have been much more convincing if we saw it uplift and cheer.