Imagine how you'd attempt to describe it without knowing anything about chemistry or volcanism.
Not saying the 8 survivors didn't know what's up, but stuff like this is definitely how religious stories get formed in general.
It's like how the smart thing to do is run to the side of a tree that's starting to fall over (perpendicular to it's projected fall).
A good way of describing it to the locals might be that 'bad gasses come from the lake and cling to the ground, like a leaden miasma from hell'.
Wiki tells that one loses conscience after couple of inhales of CO2.
I assume that those folks went on high ground just after they've heard sound.
Not to say you could have figured out the carbon dioxide part, but you could have developed an understanding deeper than "god" or "spirits".
"I mean, you don't have to know about quantum mechanics, but honestly thinking that there's some magic invisible energy causing the universe to expand? I mean seriously, I think I probably could have come up with some kind of explanation better than 'Dark Energy.'"