The TL;DR is two guys, in prison for making meth, learned how to make yogurt from prison-available ingredients, and it kept them going through the hard times in prison. Now they're out, they don't make yogurt but they are staying straight. It's a character story, not a business lesson with a three bullet-point redux.
And, in fact, paragraph two is the summary you were after:
> They don’t know it yet but that moment will change their lives. Not only will they go on to make yoghurt using heat from the radiator pipes running through their cells, they’ll turn yoghurt into a prison trading commodity so popular it will be smuggled between New Zealand jails. Wal and Dylan will become known as The Yoghurt Mafia, and their new obsession will give them a reason to carry on in a relentlessly bleak environment.
I'm not sure where your "if only" came from, but it's unjustified. Back in your can, Oscar!
They did not even offer a summary in the beginning.