The most basic principle of OOP is the bundling of behavior and data. If you don't have that, you don't have OOP.
PODs + independent functions is not OOP. ECS-style designs are not OOP because ECS-style designs are PODs + independent functions (it's a relational model). OOP and the relational model are not equivalent, if they were the object-relational impedance mismatch wouldn't exist.
Polymorphism exists independently of OOP. OOP is not the only way to model relationships. OOP is not the only way to encapsulate data. OOP is not the only way to abstract things. Sometimes it is by far the best way to do all three. That's when you use it, not because of some silly attempt at purity of style.
People have a really distorted view of procedural-style programming where it's all global variables and copy pasted code. Couldn't be further from the truth. Even within OO codebases you have tons of procedural-style code that just happens to be stored in a class with the little "static" keyword in front. Many "procedural" codebases (i.e. stuff written in C) have lots of OO-style code with structs full of function pointers.
But it's wrong to say that it's all just OOP in disguise. If behavior and data are not bundled it's not OOP, and most code doesn't need that, not even when you need abstraction + polymorphism. Encapsulation is the main one where OOP is often the best approach.
PODs + independent functions is not OOP. ECS-style designs are not OOP because ECS-style designs are PODs + independent functions (it's a relational model). OOP and the relational model are not equivalent, if they were the object-relational impedance mismatch wouldn't exist.
Polymorphism exists independently of OOP. OOP is not the only way to model relationships. OOP is not the only way to encapsulate data. OOP is not the only way to abstract things. Sometimes it is by far the best way to do all three. That's when you use it, not because of some silly attempt at purity of style.
People have a really distorted view of procedural-style programming where it's all global variables and copy pasted code. Couldn't be further from the truth. Even within OO codebases you have tons of procedural-style code that just happens to be stored in a class with the little "static" keyword in front. Many "procedural" codebases (i.e. stuff written in C) have lots of OO-style code with structs full of function pointers.
But it's wrong to say that it's all just OOP in disguise. If behavior and data are not bundled it's not OOP, and most code doesn't need that, not even when you need abstraction + polymorphism. Encapsulation is the main one where OOP is often the best approach.