That is actually the best approach start from the low competition ones and then build up!I am working on a new blog right now and I combine both methods that you described. I aim for the low comp keywords first as I start from scratch and my website doesn't any authority. This way I will start getting traffic faster. And of course build the backlinks ones I get the first batch of content. I personally think that Income School approach is aimed for the newbies that get overwhelmed by the idea of SEO. So they excluded that part to stand out on the market and attract certain audience.