It's somewhat sarcastic because Schopenhauer states that there is no relationship between the art of debate/convincing and the truth or seeking the truth. His advice, however, is perfectly valid and can be used and, to some extent, is used intuitively, every day.

I'd recommend reading the entire essay, though - the summary on Wikipedia is very brief and also, this is the only essay written by a philosopher I've ever read that actually made me laugh.