I think he's referring to the fact that there is a certain segment of the left who, if they hear something that they don't like, or that they don't agree with, then they aggressively try to shut down debate or discussion on the matter.

There is nothing "progressive" about this tactic whatsoever.  It is tantamount to the muzzling of free speech and open debate.  The "my way or the highway" stance of some on the left is analogous to what one might expect to see in some type of dictatorship.  It certainly doesn't belong in a free and open democracy.