The original commenter answered the question of the thread: "here's a book I'm reading". They got in response a screed about "neoliberal" politics. That the response is wrong is besides the point: it was a really rude way to respond to someone recommending a book. The civil and productive way to write that response would have been to recommend in addition another, countervailing book.

His comment was way less rude and way more productive than your comment.

Agree to disagree.

He's trying to have discussion, who are you to tell people how to communicate?

Sure, it's just a totally different conversation than what the thread's about, and a super rude one. I'm not the boss of him, but I guess I get to have off-topic conversations too. "Next time, on book recommendation threas, recommend another book, instead of writing a screed about how bad the politics of some other book are."

It's an opportunity to discuss, should he create a new thread to discuss this book and maybe the same person will see this thread? Kinda weird, especially when this doesn't hurt anyone.