The experiment in the article goes further than this.

I expect a self driving car to be able to read and follow a handwritten sign saying, say, "Accident ahaed. Use right lane." despite the typo and the fact that it hasn't seen this kind of sign before. I'd expect a human to pay it due attention to.

I would not expect a human to follow the sign in the article ("Proceed") in the case illustrated where there were pedestrians already crossing the road and this would cause a collision. Even if a human driver takes the sign seriously, he knows that collision avoidance takes priority over any signage.

There is something wrong with a model that has the opposite behaviour here.

Not really, as those attacks discussed here would not work on humans.