It seems extremely exaggerated, such explosions require complicated bomb design, otherwise the first stages of the explosion throw the reacting matter away and

It seems extremely exaggerated, such explosions require complicated bomb design, otherwise the first stages of the explosion throw the reacting matter away and it won't react completely. However, 5 megatons can destroy city completely, and if it explodes close to ground, it will create lots of contamination. Then it is up to the winds. Bad wind will make this contamination a major catastrophe thousands of kilometers away.