True - just chaulk it up to another FF8 plothole. I guess the only way the writers could talk their way out of it is that the Lunary Cry is caused by the Lunatic Pandora, which seems to emit some beam of energy which the monsters travel along - either through technology or magic, its hard to say, which could cause physics as we understand them not to apply.

Also, I think the moon in FF8 is a LOT closer to the planet then Earth's, espeically given how large it looks on the surface of the planet in all the FMVs, so it might take a lot less time to transverse the distance then 3 days. Whether it could still be done in a few minutes is debateable, but potentially shorter than days.