>>50787325Because 'destroying the planet' could mean a variety of things that aren't necessarily 'this planet will be mass scattered by this impact'.
Mr. Stone in the games (which alright yes are different from the manga but the main plot event is the exact same) literally uses the term '6 miles in diameter' in reference to Deoxys' meteor's size and the wording in that arc is generally 'the world will be destroyed if this meteor hits earth', but again, of course, a meteor around that size is not going to overcome the gravitational binding energy of Earth, it's just going to end civilization/render all life extinct.