I would not be concerned, the dog felt threatend by a strange person, then the dummy chose to challenge him by grabbing his neck and stare at him. You have a perfctly normal dog. But you were right to have corrected him.
Even though he was startled, he should not have snapped. A low growl would have been fine.
You will now have to be more on your toes now knowing he does not take things lightly. Back to training so you have full control over that dog.