There are a couple sections from one of the early pages of Lesson 1 that address this concern:

The wrist is permitted, but there's a key point to this that many students tend to overlook. They often expect it to be a factor of how long the mark is being what determines which pivot you use (smaller strokes being executed by the wrist), but that's an oversimplification that can be misleading. Rather than the length of the stroke, what matters is what the mark demands. If it needs to be smooth and consistent, it should be drawn from your shoulder, regardless of how short it is. Yes, this is not easy and will take getting used to, but to explain why that's not inherently a problem you can review the section that follows on the path of least resistance.

If however the mark requires a lot of tight and sharp turns, then the smaller radius the wrist provides is more suitable. This is a factor that comes up more frequently in detailing and texture, but that's not something you should assume. Always look at what the mark you're looking to draw requires, and make your decisions based on that.