Two Questions for Professor Farnsworth--Posner

In his latest post, Professor Farnsworth says: "I expect that the Justices who decided the gun use case could have passed a polygraph test in which they said the semantic arguments were a substantial basis for their decisions." Two questions: 1. What does he base this expectation on? 2. What would their passing a polygraph test show? That semantic arguments were a substantial basis for their decisions, or that (to avoid cognitive dissonance) they merely thought they were?