Well, no. At both Hiroshima and Nagasaki people survived despite being at ground zero and people died despite being well outside the lethal zone. Vagaries of fate.
Akiko Takakura.was only a few hundred feet away, but she survived.
If you have a surface detonation of a 400 kiloton nuke no one will survive being at ground zero because they will be vaporized.
This is in contrast to Hiroshima, which was an air burst whose fireball never reached the ground.
As for people further out dying - sure, that’s always a possibility. But the further out you are the more likely you are to survive. It’s just not guaranteed.
Intensity decreases at the square of the distance. Meaning something 2 times as far will have an intensity 4 tines less.
If you have some shielding than that greatly increases your survival chances,