But I interpreted his question more on terms of: "How much should I expect to learn about religion through historically relevant philosophy books? If I study philosophy, am I also studying religion?". And the answer is: You will hardly learn anything about religion in philosophy books unless you read the works of someone like Aquinas.
I understand what you are saying and it is true that most philosophy won’t teach you all that much about modern religion, but I think the historical overlap is more than you give it credit for. Even more recently with people like Nietzsche and Schopenhauer they certainly have insights on religion.
I was only replying to the last question how intertwined they were historically, where you were replying to the the whole post. I don’t disagree with you in general though.