The statement is an understatement since he claims he "does not write all that much." This kind of understatement in his brief autobiographical essay reflects a tone similar to that found in "There's a Man in the Habit of Hitting Me on the Head with an Umbrella."
An understatement serves as a literary device where authors portray an occurrence as less significant than it truly is.
They are simply bystanders.
The proper line is: "I hear America singing, the varied carols I hear,"
After stating that everyone sings, he lists the different voices that together form the American song.