It's a myth, but like most, it's partially based on facts. It used to apply to mealworms, in the 80's, before superworms were around and popular.
I've seen it with superworms, in one situation, about 15 years ago.
We had an african flat rock lizard (anyone know where I can get one - he was beautiful). He escaped and ended up in our walls for a few months. Shortly after we caught him again, he died (I'm guessing he ate some poisoned oriental cockroaches... we had a few of them in the basement... hate those things...).
When I found him, there was a superworm boring into his body. HAd I got there a few hours later, I would have thought it bored OUT. Fact is, a dead lizard is a great place for a beetle larve to chew into - especially superworms, which will clean a skeleton almost a fast as dermestid beetles, but not nearly as neatly and delicatly.
Someone must have saw a similar scene decades ago, and assumed the hard-shelled insect was swallowed whole and came out, not in.