The pages show that OOP actually hinders the separation of interface from implementation rather than promotes it. Message passing is a term most often associated with Smalltalk. A good paper to read would be The Smalltalk-76 Programming System Design and Implementation by Dan Ingalls. It describes how message passing worked in an early Smalltalk implementation, and gives a pretty good description of the overall language. It doesn't make you sound stupid to ask.
Message passing is extremely powerful.