It's true but weak at the same time:

• Have you read bible/quran and the other books in christianity/islam? No? Then you can't criticize it!
• Have you read at least some books about astrology?
• Have you read books about flat Earth?
• Have you read Mein Kampf?
• ... and so on

It's true that if you want to have a deep down discussion about Marxism, you need to read the book, there's no way around it. But at the same time it's unrealistic to expect that most people will read at least some books for every ideology to form an opinion