A subsurface dweller would always see and feel physical bounds and would probably assume that's all there was to it.
Otherwise every discovery would have the potential to discombobulate them under the right circumstances.
Personally, such an argument would be a bit simplistic, but you'd be a fool to ignore how discombobulating evolution has been to much of Evangelical America. People who believe they are the center of universe tend to find it difficult to accept evidence to the contrary. To use another literary reference, there are a lot of warty bliggens in the world.
I guess my point is any large enough group of beings will have a variety of responses ranging from embracing new ideas and information wholeheartedly or rejecting them out of fear and ignorance and every possible combination of response in between. And it can be tricky to figure out which sub-group should represent the whole in these sort of blanket statements about how "they" would "feel" about an arbitrary bit of knowledge that they couldn't have foreseen discovering.
EDIT: It’s online! http://www.astro.sunysb.edu/fwalter/AST389/TEXTS/Nightfall.h...
It was less than a century ago that we learned that the universe was a hundred billion times more complex than we thought it was.
Imagine what we don't know now.