This argument is faulty, implying that Gates' time is more valuable than others'. We need both time and money to survive, when you need not worry about the later, logically you can approach other pursuits with more dedication.
He makes the point that despite his time being so much more valuable to him that he is still willing to spend his time doing good things instead of just having a stress free easy life.
That is the case for everybody, for time is limited and money can be created arbitrarily.
Don't take my comment as an attack to the OP, I'm just stating that the situation is the same for everybody, and there are many people out there with less resources, but I claim their contribution is as valuable, if not more.
For a billionaire like Gates, he effectively only has one currency left to worry about, since time is the only thing that's important and worthy of consideration for him. That is why I said time is more important to him than it is to rest of us, since he is unlikely to trade his time for anything other than what he truly desires.
I argue that time is always worth more than money. You can transform time into money, but not the other way around.
> which is why the majority of us spend most of our days working for someone else
You can use time, money or both at the same time. In your example, you have both every day: first money, then time.
If you work for someone else, as almost all people do, you have made the decision that the time you spend at work is less valuable than your paycheck.