Of course, I don't know of many other countries that originate from a slave revolt, so in that regard "first" may be fairly meaningless. It's also possible they're just referring to cases of colonial independence in which the newly independent state was not primarily inhabited and dominated by European colonists, as was the case with the United States and a fair number of Latin-American states. The situation of the white colonists in the American colonies may have been such as to warrant revolution, but it certainly wasn't comparable to the situation of the indigenous inhabitants of most European colonies both before and after that time.