I'd say the distance in terms relative to communication rather than geography is bigger between the Undardark and surface than between the Sword Coast and the Frozen North.
In Badur's Gate games, the Underdark is always like 1 small dungeon away from the surface. The Zhents have literally built an elevator to smuggle stuff like captive beholders in and out. My point is, it's not too far-fetched for a druid (who's supposed to be an expert in all things nature) to know what a deep rothé is.
