Yeah it's a bugged dialogue trigger which should only be available to those who raided the Grove,
Yeah... I never once raided the Grove and I still got that dialogue on my last play-through. I always thought it was because there might be a way to get to Moonrise before you get to Last Light and to meet/observe/overhear Marcus there somehow before you ever meet Jaheira. But ... that would probably be impossible since you wouldn't be able to withstand the shadow-curse if you went straight to Moonrise (no blessing from Isobel and certainly no Moonlantern/Pixie-blessing).
S.
