While he puts "intellectual property" in quotes, he doesn't actually say anything about why it isn't property. There is no single libertarian theory of intellectual property; Rothbard, for example, claimed that some copyrights are legitimate. Rothbard's example was a mousetrap designer stamping his mousetraps "do not copy," and in Rothbard's view, this constituted a contract as the buyer consents to the condition. Other libertarian theorists have offered alternative views, but I'm not familiar with many of them and don't yet have a strong view on the subject. I tend, however, to not believe that these things constitute property.