When you think about instructions for something like this, look at the motivation of the seller. They want to sell more BSF larvae, not necessarily give you information for the most nutritious BSF larvae. If you are not really careful about how you feed them, you will end up with a really disgusting stinky soupy mess. Compost goop gets on your hands and is incredibly hard to wash the smell off. The producer does not want you to have that stinky mess because you will never buy them again, so they say they don't need to be fed. They will live a long time slowly breaking down their own body in order to survive. The process is called catabolysis and you end up with a feeder insect whose nutritional content is depleted.
They like soft moist foods best. I would never feed them any gutload that is grain based since I want to reduce the chance of aspergillus growth. They so not seem particularly fond of things like kale