He added a lesser option, catastrophically harming humanity, so whatever he meant by the first is immaterial (“there’s a 70% chance of a hurricane or strong winds”). Furthermore, if it wasn’t a high number chosen for dramatic effect the estimated percentage would be completely arbitrary.

I disagree that 70% was chosen for dramatic effect.

No, you’re right, it was chosen because “trust me bro”.

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.