"Shade some cells to leave behind a single, connected group (the cave), with no enclosed, shaded cells. In other words, all shaded cells must be connected by other shaded cells to an edge of the grid."
This plus the flip of what's shaded in the example led me to think that what I wasn't shading were the walls of the cave and because they were unshaded, the walls were allowed to be enclosed. I should've tried to solve the example - I would've noticed there wasn't a unique solution with that interpretation.
Going back to the competition puzzle, this is making much more sense now, thanks. :)