I agree that American exceptionalism was there from the very beginning, but I think that concept was used to justify a narrower set of long-term goals. Those goals changed as some were achieved and the original message being adapted in the face of new challenges.

The idea seems related to start-up culture as well (think "two guys in a garage", and "we're different" as well as "war stories from back in the day" during shareholder meetings).

Time will tell if setting ourselves apart sets us up for failure in the long-term.