Well, I don't want to speak for David, but I would guess it goes something like this: Your priorities are what they are, no matter what number or letter you attach to them. They come out of your heart and soul and into your life, and not out of your calendar and into your life. You will live out what is truly important to you - even if that is avoiding work - whether you "prioritize" or not. So what needs to happen is not to force yourself into a predefined list of priorities, but develop a system which allows your "intuition" i.e. your heart and soul to choose what is truly important to you in the moment, knowing that your trusted system will keep you informed as to possible options and requirements.
As Blaise Pascal said: "All men seek happiness. This is without exception. Whatever different means they employ, they all tend to this end. The cause of some going to war, and of others avoiding it, is the same desire in both, attended with different views. The will never takes the least step but to this object. This is the motive of every action of every man, even of those who hang themselves."
Managing your priorities implies that because you attach "A1" to something, it will be done first, and is the most important thing to do today. The reality is that a single call from a single person can completely nullify all of the "managed priorities" on your list. Anyone who has received an unexpected call from the ER knows what I mean.
You have priorities, but you can't manage them in the sense that you can't control or determine how they will play out. They change as new information makes its way into your attention. I feel that saying you are "managing your priorities" simply means you are believing you can determine in advance the order you do the things in your day. I feel that "having priorities" is more of a high-level awareness allowing for the constantly shifting nature of the things you have to do.