It does mention Slate, among others. After skimming the intro, I was similarly surprised it didn't mention Self, but it was probably more directly influenced by Slate. (Slate and Io were both strongly influenced by Self.)
Self and Io are both languages that have similar concepts but weren't direct inspiration; in Self's case, it was pure chance (I've never actually used Self), and in Io's case, Slate was a more direct influence. Io didn't have much mindshare, despite being another language I love (but never use).