I think people are reading this too literally. The 'one thing' isn't your task, it's your goal. You engage in whatever tasks are necessary to achieve your goal

I think people are reading this too literally. The 'one thing' isn't your task, it's your goal. You engage in whatever tasks are necessary to achieve your goal in a way that is good for the business. This ensures that each business function is the best it can be. For example, suppose the next most important thing for your business to do is to reduce marketing spend. you make that someone's goal, and that person focuses on reducing CPA for several weeks. obviously, you don't want this person to divert their time to answer support emails--you want them focused on saving your company boatloads of money.