The real world doesn't care nearly as much about effort as it does about the end result, except as a mitigation for failure. Why effort and not .2 x effort + .8 x accomplishment?
To answer your question, because no matter how smart you are, there will come a time when you HAVE to work extremely hard. If you don't learn this skill early in life, you will suffer simply put.
The message is the culture of effort, and I could not agree more. If you "teach" your kids to optimize their time, for them a test score of 8/10 would be enough if they need to dedicate 5 times less time that they would have to get a 10/10. Eventually this will derive into decreasing more and more the effort and, with it, the results. Knowing when to optimize is also a learned lesson, which comes (again) from effort, failures and success.
I have felt what they describe in the text, and... I wish somebody would have praised my effort back then. If we check most people that have accomplish something in their lives, they have done through effort. Yeah, sure, more variables are needed (you can't run like Usain Bolt or think like Gauss only by effort) but if there is a common characteristic, this is effort.
Truly life-changing opportunities are fairly rare, and they usually pop up when you don't expect them. To capitalize on them, you have to be prepared. If you've been sitting around slacking off for the previous several years, you're probably not going to be in a position to pounce when the opportunity arises.