As for the second part, there's a lot there. It's good that your personal anecdotes you don't know of anyone held captive, but I don't think that was the question, and again I feel like that's coming from a place of wanting to generally dispute that people should be concerned about the practice of holding relatives captive to punish dissent.
For the rest of it, this is exploring the possibility, illustrated through Cixin Liu in a hypothetical example, of what might happen if he ever hypothetically wanted to extricate himself from China. His international success brings with it questions relating to free expression in China, so I treat these conversations as not terribly surprising, and frankly quite necessary.
And again, I don't think there's a literal case that he wants to do this, but that wasn't the point, and your confusion there again seems like it's coming from a place of generally wanting to deny that there should be any concern with how China could conceivably take family members hostage to pressure expat dissidents. And again, I could not disagree with that more.