Since you are bringing up WW2, I'm guessing you do not realize that by "destroy humanity", the OpenAI insider (Kokotajlo) means "reduce the human population to exactly zero". He does not mean, "kill people" or even "kill many people".
Look, it may well be something he believes, and he’s free to prognosticate (or market) however he likes, but I see absolutely nothing to support the number outside of his own opinion.
Besides, there’s no time limit on p(doom), so it’s completely unfalsifiable (“on a long enough timescale…”), and it’s about the destruction of humanity which means it’s unprovable as well. That, in my view, makes his 70% guess a sensational statement lacking scientific merit.