Underground earthbag shelter

Very good question.

Generally it's like anything underground, you either make a barrier with say 2 layers of plastic or you put a membrane such as bitumen or better still both. Waterproofing is not to be underestimated as you rightly point out. Of course the people depicted did neither, probably didn't put in drainage on the slope either so as to divert water around the structure and will eventually realise that they wasted all of that work as the moisture gets in

If I were doing the same [and I'm still learning about it] I would have put in the plastic, french drains, covered in bitumen on the outside, backfill with gravel especially down low to allow better water drainage, and probably did a concrete slab for decent foundation. [Trying to understand how gravel foundations work currently]. I'm also no expert on engineering and would want some proper opinions there, but I like the idea of those poles as it heavily reinforces the roof well beyond the structural needs so as to maintain strength. The roof to me is the trickiest part for below ground.