True Language

As asked by someone:

My question involves and article that I read from the Ohio State 
University Language Files. In this article, it lists three “design” features 
that all communication systems share: a mode of communication, 
semanticity, and pragmatic function. Then it goes on to list the qualities that 
some, but not all, communication systems have, such as 
interchangeability, cultural transmission, arbitrariness, and discreteness. When 
explaining interchangeability and cultural transmission, the article gives 
examples of these aspects in non-human communication systems. It uses an 
example of silkworms’ chemical communication of the female sending and 
the male receiving messages indicating readiness to mate to explain the 
idea of interchangeability. Finally, the article goes on to list the 
two defining characteristics of what it calls “true language:” 
displacement and productivity. Now, my question is why are arbitrariness and 
discreteness not listed as defining characteristics of human language. What 
non-human communication systems argueably possess these qualities?

Can anyone help answering it, since it's beyond me, I'm afraid.