How do you distinguish between a superclass that always returns null/noop, vs. a subclass that happened to return null in this specific case?
Sometimes this is useful/vital for setting expectations to the user what functionality the instance is likely to have.
Now, you could refactor everything to have the superclass's implementation either throw a NotImplementedError, or return a sentinel value... but changing every call site might be a gargantuan task. Similarly, adding static metadata to every subclass might not be feasible. But checking whether the function's (pre-bound) identity is different is a very easy hack!
Ironically, this might indeed be exactly what Apple is doing here, and they're doing it in a tight loop.
