The section "International Game Theory" does not seem to me like an argument against AI as an existential risk.
If the USA and China decide to have a non-cooperative AI race, my sense is that this would increase existential risk rather than reduce it.
Why would Knightian uncertainty be an argument against AI as an existential risk? If anything, our deep uncertainty about the possible outcomes of AI should lead us to be even more careful.
Not sure what the author's argument is, but here's my interpretation: AI risk being a Knightian uncertainty is an argument against assigning P(doom) to it.