Think of poison like a bullet - it hits certain parts of you very precisely, taking out your stomach or your kidney or your heart. Radiation is more like a fog of death - it attacks every cell in your body simultaneously.
That's true! But the same could be said for a bullet, or a grenade, or a landmine, or a car, or...
If you only look at lethal hits based on whether they kill you or not, a cannon is the same as a bone getting stuck in your throat. Either way you end up dead.