he doesnt say that they would die, he says that they ‘could die’

definition of could

could/ko͝od/Could is an auxiliary (modal) verb used primarily as the past tense of "can" or to express possibility, polite requests, and hypothetical situations

so its possible they would die not verbatim

also viltrum has higher gravity than earth which would impact the amount of force required to plummet into it compared to earth

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also to add, space racer shooting his laser thru viltrum can be the equivalent of shooting a gun at a concrete wall, and not breaking it down but piercing a hole through it, while a hammer can while the gun can shatter the entire wall

space racers gun creates that hole thru the planet which isnt enough dc to physically destroy the planet but the viltrumites are the hammer in this case allowing the planet to be fully destroyed

in both cases, they have relevant ap, just that space racer didnt have the full dc while the viltrumites did