secret wrote:" Is John ill? He's lost a lot of weight."
"Yes, he is rather slender these days, isn't he?"
I'm not surprised you're having trouble answering. It sounds like a perfectly normal English conversation to me, without getting into the philosophy of whether people actually answer other people's questions. (In other words, the answerer hasn't said whether John is ill or not, which is the real question - but that's life, that's how people "converse".) Grammatically, there is nothing wrong with the exchange.
Your explanation by the way doesn't hold any water. We constantly use question tags in this way