Pretty sure he is implying that the contact areas of plastic wads go from a solid to a liquid in the presence of combustion gases and pressure that occurs in a barrel at the moment of firing. My bronze brushes and old T shirt patches give me some indication of the same.
That liquid would offer up less resistance to being blown out the end of the barrel than felt wads. Less resistance would further nullify very subtle differences that exist only theoretically to begin with.
Or, not.

Best,
Ted