The author specifically calls out why just being sent notes doesn't help, when some teachers do exactly that.
The advice is to understand, and consider questions while making notes. He has a section on when to ask questions in class.
The very next line after the one you quoted explain why he errs towards the former, but it does not say you should not concentrate, you should not try to understand, and you should not ask questions.
There is absolutely time to do all three. He even suggests ways of making notes in a condensed way. If you're actively listening, thinking about what you're writing, you're dealing with the level of understanding appropriate to have in that moment.
The advice is to understand, and consider questions while making notes. He has a section on when to ask questions in class.
The very next line after the one you quoted explain why he errs towards the former, but it does not say you should not concentrate, you should not try to understand, and you should not ask questions.