You and chromatic are defining polymorphism differently, and then disagreeing about whether his example is polymorphism.

But the thing is that his implicit definition is both more useful and more standard.

Polymorphism is the ability for different data types to go through the same code and automatically be treated appropriately. One way to generate different data types that you might do this is to have a class and a subclass. (Or to have 2 subclasses.) But there is, in general, no need for them to be related in any way. It is just often convenient for them to be connected.