I don't understand this line of criticism exactly. By putting new information in the context window, you are materially changing the activations at your point of sampling, which is literally "customizing with mere markdown files."
Taken to the extreme, the attitude that there is some special incantation that will unlock all capabilities is silly, and a lot of the "prompt engineering" discourse is similarly kind of dumb, but in-context learning is clearly a real thing.
even if that works one time you can never be sure that your customization is in place or fell out of context's important zone or is contradicted by later context . you've reverted back to base llm behavior.
they are being sold as more powerful than they are. Like llms are intelligent blank slates that can be customized with mere markdown files.