Because he's completely full of crap. During the 1990s he was a strong advocate of U.S. military intervention in the Balkans to topple the Serb government and separate Kosovo from "greater Yugoslavia."
Documentation? I’m not saying it is not true, but I thought I was following Serb/Kosovo situation fairly closely in the 1990s, and I don’t recall hearing of John Bolton much before when he was nominated for the UN post.