Not sure, I live vicariously through the reports of others.
From the autonomy presentation, I though Karpathy was talking about detecting non-signally lane changers, which implies they detect signalling lane changers.
I don't think the car sees other car's signals. Many times, when in AP, I see a car signal ahead of me as they are obviously going to move over into my lane. The car takes 2-4 seconds longer to start to slow down than an observant /courteous driver would take. It seems that the car ahead needs to be ~50% into my lane before my car starts to slow down.