Is this an example of American exceptionalism?
More an example of American wankism.
That they're "both Americans" is hardly a surprise since it was a poll
of Americans.
As the poll site notes, "The open-ended nature of the question produces the names of people with high top-of-mind name identification, most often hailing from the world of government and politics"
--- in other words it's worth about the same as when your local entertainment rag runs a poll on "the best injury lawyer in town". Respondents will simply check a name on the basis that "I've heard of him" rather than actually having any experience on which to base that choice.
How else to explain the presence of high-placing names like....
Queen Elizabeth... (why?)...
George W. Bush...
Oprah Winfrey....
Sarah Palin...
If the poll were honest it could have just asked the question, "who have you heard of" and gotten the same answers.