I'd expect he thinks that people are capable of realizing that AI is very dangerous and also that democracies end up mostly representing the will of the people, from which it follows that Hypothetical AI Leader Australia would agree to ban it too. This argument doesn't work for countries which don't care what their citizens want, like China.

I do think that it's a questionable decision to alienate China this much in this essay, instead of leaving open the possibility of China agreeing to a treaty that'll limit their progress. I suspect Dario is doing this to signal his allegiance with the US government, in hopes to increase the chance they'll go along with him, which is an unfortunate choice but plausibly the correct one.