A similar bad example would be using familiar round objects to teach equivalence: there are 10 apples, and 10 oranges, so apples == oranges.
Car's don't extend vehicles, and duck's don't extend birds. Just because we can organize these physical things into conceptual hierarchies based on their functionality, doesn't mean we can use them to understand polymorphism.
As to polymorphism, that comes for free from understanding language. Most people will understand a "No vehicles allowed" sign to prohibit cars but not people. What else is that but an understanding of polymorphism in evaluating a predicate?
Object-orientation works because in some programs, cars can be treated as if they were a vehicle, without knowing what kind of vehicle it is. This is the principle of substitutability, and it's the actual reason that inheritance hierarchies work. But in other programs, cars and other vehicles may not have much in common and should not be part of an inheritance hierarchy!
The real-world ontology is not a reason in and of itself to create an inheritance relationship.
The point is the commonality of mechanism, not the commonality of any specific ontology. And when you're teaching, that's enough for one lesson.