So probably the author put a "distance less than or equal to" constraint. Having a "distance equal to" constraint would have the point always on the circle. And, magically, "distance larger than" would have the point always outside the circle.

That one didn't show code. I have to assume the code tests if the point is outside the circle before constraining it, similarly to how a later constraint tests if a point is inside the circle before constraining it (the one where you shove balls around).