But this is simply wrong. The error creeps in in the second assumption:
> An agent can analyze another system, even a complex one including other agents, using quantum mechanics
This is likely a good assumption, but quantum physics would tell Alice that the coin's atoms are all firmly in a state corresponding to either 'heads' or 'tails'. The author is actually making the following assumption:
> What we classically think of as chance is completely interchangeable with the unpredictability of quantum superpositions.
This notion has already been proven wrong many times over.