I pop adults all the time, and have never caused an injury. Every breeder I know pop adults all the time. It's how we verify whether an adult animal we are interested in or have purchased is indeed the sex it is represented as.
I don't even probe any longer. Popping is my primary way of sexing all my ball pythons. I don't carry a probe kit with me to the pet store or a show.
So the fact that this gentleman tells you he can pop an adult does NOT mean he's full of crap. There's a HUGE difference in what you read in a book and real world experience.