Since when?
It's not certain Cavendish would have died. In a risky situation like that one(surrounded by bombs everywhere), it's better not to try to play the hero. Doesn't mean however that if it actually hit him, it would have killed him. Just like how Gladius thought his ultimate move would have killed Bartolomeo, but he survived just fine. In the latest chapter, Zoro even said he'd die if he fall from a mountain level distance when in skypiea he survived falling from a larger distance. Hell even in the current arc, someone weighting thousand tones survived from falling from the bird cage