Whitman has just given us an incredible portrait of an America that is and
also of an America that he wishes would be, so this admission near the poem's
completion describes both the nation and the narrative voice (which, let's face it, is Whitman, or at least his idea of himself -- the poem is also known as "Poem of Walt Whitman").
There's nonchalance in Whitman's understanding of contradiction: who cares, he says, that he may not be consistent...very little about life is. In that attitude he was joined by at least two of his contemporaries. Ralph Waldo Emerson opines that "a foolish consistency is the hobgoblin of little minds." Among other things, Emerson is arguing against those who value sticking to one's guns above re-evaluating and getting things right (like the idiot pictured on the link). Emerson and Whitman are joined by one of the foremost philosophers of the 19th century and a man hated by the far right (look up Eagle Forum and harmful books -- I don't like to link to fascists), Friedrich Nietzsche, though many attempts were made to co-opt him.
One of Nietzsche's aphorisms from Twilight of the Idols reads: "I mistrust all systematizers and I avoid them. The will to a system is a lack of integrity." The idea of a system is inextricably linked to closure: its laws must explain completely or the system is inadequate. Understood on a basic level, there are plenty of "gambling systems" available to Las Vegas aficionados, but to date, the casinos still rake in massive profits. Something obviously is escaping those systems.
So here we are on the day of election. Maybe Whitman is smiling amazed at the transformation in American society from his time to ours.
Or maybe he's simply saying it's about time.