Mao mentions the essentials of a topic. He isn't saying "you must be an expert on something before you can talk about it" rather "You must understand it before you talk about it."

Someone who knows nothing about a topic cannot contribute to a discussion on it, but someone who knows at least a little still can. Look at the libs that wander in here sometimes, they are the perfect example of "talking nonsense" that Mao describes. They contribute nothing, because the things they say are said out of complete ignorance of the topic.

Basically, you could sum it up as "Check yourself, before you wreck yourself." Examine your own understanding of a topic to work out if you do actually know something about it, or if you are just spitting hot air. If you think you do, but others inform you that you don't, that's not a bad thing, it's an opportunity for learning.