Never had a caver without helmet in any of my trips. Taking it off is big no no and I have seen people going smaller holes then this one. My guess is that this cave wasn't too long and he just wanted to get in fast.

To be clear, there is a big difference between "spelunking" and real "caving." I haven't met a caver yet who like being called a spelunker.

Nope, there's another NatGeo video on youtube about this cave and the cavers who discovered the bones. They all wear helmets.