This game was released after Pocket Monsters Red and Green (February 27, 1996), but Game Freak released Pokémon Yellow (Pocket Monsters Pikachu) after BUSHI Seiryuuden: Futari no Yuusha in September 12, 1998. If you compare the first boss music (Madake apparently) with the unused track, they seem pretty much identical. Footage of the first boss (starts at 0:49), compared with the unused track.
I'm under the impression that this game was quite popular, seeing as Game Freak had a whole section about it on their website since May 1997.
Edit: I uploaded it from the ripped soundtrack.