You know your are in the presence of a shibboleth when you see something being repeated again and again that adds absolutely nothing to the substance of what is being talked about. One modern instance of this is the frequent dropping of "our monkey brains" and such into discussions of almost anything to do with human cognition. This happens often over at Language Log: consider this post. What in the world does mentioning "plains apes" add to the discussion, except a shibboleth? Nothing. It is not as if we could see that if we were culturally-evolved forest lizards, we would have no such problems. It is not as though people prior to the formulation of the theory of evolution were unaware of human cognitive difficulties. And it is not as if we have the typical scalar-predicate-handling ability of the average ape, which is exactly zero. No, quite the opposite: the real surprise if not that an ape is using scalar predicates badly, but that it is using them at all.
No, this is a intellectual-content-free ritual: Liberman is just showing his membership in a club here, no differently than if he were using a Masonic handshake.