I believe Andy's point is it is better to avoid self-consciously inserting a message into storytelling, or else it comes off as preachy. By avoiding this, what is important to you is then inherent in the writing without the preachy element. (Indeed, I'd argue what is important to Andy is inherent throughout The Martian, concerning the triumph of human endeavour through scientific application, his clear disdain for power-hungry bureaucrats and those who put political expediency ahead of doing the right thing, etc). I've written about this principle in more depth here, if you're interested. :)