Edit:

So it's kind of interesting, this test (and the functionality behind it) exists specifically to make sure you can't rely on it having a stable identity -- and therefore have to omit it from the dependency array. But it otherwise behaves the same as the popular user-land implementation involving refs (which you still have to include in the dependency array, because it works by providing a stable identity).

So there's some internal trickery in React to make sure that even though the effect event is recreated and has a new identity on every render, any time it's invoked you're accessing the latest inner callback.