From your framing, it sounds like the advice is to start with a person who lacks motivation, and then tell them to be disciplined, and it's sufficient for lasting change. In this framing, the advice is absurd.

But it's far more interesting to consider how the one phrase relates to the rest of Willink's perspective. For example, he argues that that you can frame discipline and hard work as a method to be physically healthier; care for your family better; or help you earn more income or career capital.

The phrase is an introduction to the rest of his arguments, and it isn't quite as interesting to debate its merits in isolation.