This point has been argued far more eloquently and accurately by Cardelli and Wa...

This point has been argued far more eloquently and accurately by Cardelli and Wagner, "On Understanding Types, Data Abstraction, and Polymorphism".

See Section 1.3 "Kinds of Polymorphism" (there is a typo in the paper and section 1.4 is mislabeled as "1.3" as well.)