IMO this would be a terrible idea to let the robots park cars in the same car park as humans. They most likely have a dedicated car park for this where ever car is parked by a robot.
The article says:
"A Stanley Robotics autonomous parking robot then slides underneath the vehicle, lifts it by its tyres and transports it to a secure storage area."
The article says:
"A Stanley Robotics autonomous parking robot then slides underneath the vehicle, lifts it by its tyres and transports it to a secure storage area."
Which seems to suggest this.