I haven't a clue honestly. In all the years that I've played it, I never did find any hidden rooms nor did I really ever find out why that was placed in there. I think what you said is probably true though, since the Arcade version had that hidden room and this one did not (but then again, a lot of things were changed from the arcade version when ported over to the NES... which was okay by me).
While this doesn't explain anything, it might be interesting to note that very same music track is played during the credits of the NES Rainbow Island port (again, very different from the arcade, but moreso this time around, and the American localization was even more different by censoring things like crosses and whatnot).
So your guess is as good as mine. --- Richter Belmont- Die monster! You don't belong in this world!!!! *Flushes Toilet*