From Willink's perspective (interpreted charitably), there are many situations where it's useful to "just do it." For example, say you want to do a task, but you want to get comfortable first because you're anxious. You can watch a funny video or browse the internet to calm down, read a motivational blog post, and do various chores to 'get in the mood.' Then hours pass by, and you still haven't started the task.
A better solution is to start with the action causing anxiety, expecting the shift in mood to follow the action (before trying to change your mood first), which is supported by recommendations in cognitive-behavioral therapy (CBT). Actions first, mood follows is a good, actionable insight supported by CBT counselors.
> "For those who lack enough motivation, which is the situation the advice is supposed to address, I don’t see how it helps."
If you're disciplined despite lacking motivation, you can improve your physical health and earn better career opportunities, letting you earn more and have more freedom. Therefore, by focusing on actions more than motivation, you can improve your circumstances. This is a charitable interpretation of Willink's perspective.
However, my personal view is moderate. Motivation does matter, because it reduces the amount of willpower needed for work, and lets you choose the right tasks and goals to work toward. Though in any case, action before changing your mood is perfectly actionable advice.
I was calling out the GP that the mere insight that discipline is usually necessary because you can’t rely on being consistently motivated fits the pattern of an insight that doesn’t by itself makes it easier to solve the problem. The insight can explain lack of success (inconsistent motivation combined with lack of discipline), but it doesn’t tell you how one might engender a lasting change. Also, the difference between discipline and consistent motivation is arguably small, because you somehow have to consistently motivate yourself to build up and maintain a discipline.
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.
I guess that for some people, being told “there’s no way around just doing the things even when you’re not motivated” by itself creates sufficient motivation long-term to do the things even when they are not otherwise motivated. But for those for which that doesn’t happen, there isn’t much actionable advice.