Their understanding of bitcoin's consensus is so primitive they can't formulate coherent criticisms. They are starting with the wrong premise. So the answer doesn't even register with them as sufficient. It's like the question "how can mail over the internet work, if there's no way for the sender to know the recipient's computer is turned on?"

The question is presented as a reasoned criticism of email. But in reality it shows a fundamental misunderstanding of the internet. The question is so misguided, it's not even wrong