By pure chance and blind luck, this man had discovered an entrance that had remained hidden from man...
...
From the pictures it looks like there are multiple entrances, some huge. I find it hard to believe that nobody knew about them.
Sounds like this is a remote area. Maybe one of our FReeper VietNam vets knows something about the vicinity.
All in all though, I tend to agree with you. Somebody must have fallen in at one time or another.