Not all financial institutions are controlled from the US either, there are a number of Middle Eastern banks that trade with Iran, and wire transfers could be organized. In addition, not all AdWords advertisers are based in the US or Europe, many are in the Middle East and Asia. I'm sure those advertisers would be quite happy to accept business from Iran.
However, I'll take your point that Google is an American company and won't break embargo. Yet I still have an uneasy feeling that Will wasn't just talking about Iran, that perhaps other nations we do trade with pay less to AdSense publishers. I'd hate to find out for example that Google pays less for Mexican clicks than Spanish clicks, or that an ad on my site from a language school in Argentina pays me less than a similar school based in Barcelona.
This geopolitical thing Will alludes to suggests he suspects or knows that this is the case outside of rogue nations like Iran.