Oh dear. That java tutorial just reads wrong to me but that's probably because I've been burned too many times by conflating polymorphism with code reuse in the form of inheritance.
I think the working programmer doesn't know or care though. They care about whether you can reuse code in some way. And whether you can substitute multiple implementations of an object act as one particular type.
The specifics of how you reach either of those goals is all they want to know and the pedantry that us language geeks love to engage in just blows right past them.
It's possible we've just interacted with different people; I've spoken to a lot that assume inheritance, specifically, and we get questions about it often.
I think the working programmer doesn't know or care though. They care about whether you can reuse code in some way. And whether you can substitute multiple implementations of an object act as one particular type.
The specifics of how you reach either of those goals is all they want to know and the pedantry that us language geeks love to engage in just blows right past them.