Why do we need the isPrototypeOf at all?

this page states:

Note: isPrototypeOf differs from instanceof operator. In the expression object instanceof AFunction, the object prototype chain is checked against AFunction.prototype, not against AFunction itself

Ok I don't really get what they are trying to tell us. Isn't object instanceof AFunction exactly the same as `AFunction.prototype.isPrototypeOf(object)? or am I wrong?

Why do we need the isPrototypeOf at all?

If i ever need to do p.isPrototypeOf(o) couldn't I just do o instanceof p.constructor ?

Addtionally, is p.isPrototypeOf(o) functionally equivalent to p===Object.getPrototypeOf(o)?