The statement author makes that polymorphism is belongs to OPP is incorrect.

Functional languages by design require better type systems that can accommodate more forms parametric polymorphism.

OPP traditionally encouraged the use of ad-hoc polymorphism (via classes) over parametric polymorphism (via generics). OPP languages generally also make it easy to perform Ad-hoc polymorphism at run-time with dynamic dispatch some languages even use dynamic dispatch by default.

TLDR: FP and OPP both have polymorphism. FP encourages parametric polymorphism. OPP encourages ad-hoc polymorphism.