I get the impression he selected 90% because 100-10=90.

But that's wrong. If the doom scenario is 10% likely, that in no way implies that the utopian scenario would be 90% likely. They're both at the extreme edges of the bell curve, so should at least have the same made-up odds as each other.

The reason I pointed out the over-the-top BS nature of the 90% figure is because it indicates that the essay is a propaganda piece and should be treated as such, rather than any kind of serious think piece.