He drove off, and someone else got the spot.
In practice humans frequently neglect to signal in instances where it is legally required (I don’t know if it is strictly legally required in California when parallel parking, but it’s certainly good common sense). Human drivers know that people don’t signal and account for that.
In this particular scenario the reverse lights would be a crystal clear indication of the driver’s intent, even if they failed to signal.
I don’t want to assume the intent of your comment, I don’t know what you’re getting at..
I have never run into this where the human driver didn’t get the hint when I remained in front of them with my reverse lights on. They have somewhere to be so sooner or later they go around me (backing up if necessary). From my understanding, the driverless car in the OP just sat there forever.