>Regardless, this has nothing to do with how the brain learns, but rather the function it has learned.

The learning rule and the hypothesis class together dictate what sort of function is learned.

>Even if a neural network somehow used entirely local learning rules, it could still be exploited with this method.

Yes, which is why current neural networks may not be the best learning method.

This method is basically just fuzzing with a really efficient method to do it quickly. But in theory you could try every possible set of inputs into the human eye, and it's quite possible you would find images like this, where only slight changes to the inputs cause entirely different outputs.

But we can't try every possible set of inputs to human eyes. So we don't actually know how fragile human brains are. I suspect that brains use similar tricks to artificial neural networks, and learn similar functions.