Nuclear bombs aren't the only technology that made ww2 so bad. Pretty much every technology has been used for harm one way in another. The question is on balance whether we are better off with or without.
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.