They don't consider Guidebooks or otherwise canon so they go with that nonsensical explanation because otherwise and I quote like everyone else "Why need Meteor then?"(Even though the entire point is that Sephiroth wants to absorb the Lifestream when it all comes up to repair the Wound done to the Planet, not destroy it completely).
That and it would destroy their headcanon of Final Fantasy just being a "Medieval series" with only okay Superhuman powers they can put against RWBY and MHA that wouldn't also be stupid if you know... games like Brave Exvius really show how powerful they are:
Hilariously, this isn't even counting the OG characters from this game