The idea isn't that it is easier to do things when not tired. It is that you specifically get tired exercising self control.

I think that can be subtly confused by people thinking you can't get better at self control with practice? That is, I would think a deliberate practice of doing more and more self control every day should build up your ability to do more self control. And it would be easy to think that that means you have a stamina for self control that depletes in the same way that aerobic fitness can work. But, those don't necessarily follow each other.