Although I don't have CD, I did some digging a couple of weeks ago about this. You all probably know about the various Demo CD + book (mooks) that have come out for PCE. These books are basically hint guides with maps, etc.
In the case of this Seiya Monogatari, the programmers(?) decided to include this shooting game. From the book, it says that the game has nothing to do with Seiya Monogatari and they just wanted to include this shooter. Not sure why they did this, but I'd guess that it was the only way to get the shooting game published in some form.
I just looked at the auction. Pretty pricey for just the CD imo. I would like to try that shooting game however.