Not a typo -- lingo from hip hop. B. Horowitz is a big hip hop fan. He invests in Rap Genius.

On a related note, I think "Birthday Song" was included at the top of the post because it tries to hide nothing. The lyrics are honest & straightforward much like how he argues feedback should be.

What makes you think it's a typo?

From (of all places) Urban Dictionary:

  clown: To laugh, make fun of, or find joy in a 
  person or event. Almost always used in a derogatory 
  or degrading fashion.