Yeah, I've heard this stupid idea also since I have some article on how to export private key on blockchain
here, and people keep emailing me on how they will recover the private keys of a watched-only address, which I'm always saying there's no way to do that. Why watched address? because..
As what OP said, they'll ask the login details of a blockchain account and after that, they will configure this
miner on the blockchain account which is really a stupid idea and I don't know how arrogant people that they still believe this method. But instead of configuring a miner, they will import an address that has a large balance on it a watched-only address, I don't know exactly the balance, and they will send all the amount of the account's balance on that address, its based on some people's emails I received lately.