I don't know about the impact thing, at least not in Eorzea. I only say that out of speculation because in the new revamped maps, clearly there are no impact craters of massive size, and anything relative to the size of Dalamud would leave a big crater.

My only guess is that it hits the other side of the planet and a scorching flame of destruction swallows the planet, or it gets blown up by the grand companies or something in the sky before it hits the planet, and the debree from the after math is what causes most of the destruction.

I'd also like to mention something someone said before that I read and was really interested in, but as a means for survival maybe everyone will be teleported into the aytherial stream until after the collision/destruction and then awaken from crystal stasis after everything has settled.