It’s obvious from this excerpt that Adams was mostly talking about British Prime Ministers, who are elected by the government and do not wield much (if any) power beyond that of being a figurehead. Of perhaps the Royalty, who aren’t elected but hold even less power and are even more of a distraction.
The US president, by contrast, is not elected by the government and has a shit-ton of power, and increasingly so as the US congress is less and less able to govern due to Republican infighting. The US president can start and win a foreign war in less time than it takes congress to even form an opinion on the matter.
Zaphod spent two years in prison for fraud, meanwhile the US president is protected by more military firepower than literal nukes and has a chain of succession longer than most Kings because the US government literally cannot function without a President.