The backpack works because it's not just black and it actually includes some adversarial input ("fake" license plates). A Vantablack poncho wouldn't protect you

The backpack works because it's not just black and it actually includes some adversarial input ("fake" license plates). A Vantablack poncho wouldn't protect you more than a regular poncho with no distinguishing patterns. The image recognition wouldn't really latch on to creases in your clothes to perform recognition. Your face would still be visible and there's nothing about solid black that further confuses the AI behind the system, there are far better adversarial examples for that.