So I checked, and no, in fact, he didn't. Americans who have won their division or absolute are:
-Jeff Monson
-Mark Kerr
-Ricco Rodriguez
-Dean Lister
-Robert Drysdale
Other than those guys, Einemo and someone named Kareem Barkalaev have won other than Brazilians. So yeah, the rumors of American dominance are highly exaggerated.
BJJ will end up like Judo. The Japanese were totally dominant for probably 20 years after Judo hit the international scene, and even now they're probably the strongest country but you don't expect them to automatically win. It'll be the same in BJJ, there will be more Brazilian champs than champs from other nations, but over time Americans and probably initially Japanese fighters will also start showing up on the podiums, sometimes winning gold. Though Judo grew a lot faster internationally than BJJ seems to be.