This is so mentally ill, clueless, and inappropriate that I had to look twice or three times to confirm that it was the real Shaun King—the Black Lives Matter supporting journalist who may not have any African ancestry, but who identifies as black for careerist reasons.
I presume he's saying that because of Malcolm X. Malcolm X (pictured right) changed his last name to X because he hated all white people and white names. (Also, his last name at birth was Malcolm Little.) Therefore the initial X is associated with black names. But in the case of Micah Johnson, whose middle initial presumably comes from St. Francis Xavier, we don't need "dog whistles", we need as I say below, what Slate called a "fog horn of a headline" when Drudge did it:
What we need is a piercing siren saying a black gunman targeted white cops because he was listening to people like...Shaun King.