If you look at the documentation, you'll see that this method is about getting the angle between 2 vectors.
In fact, Paper.js Point are actually just vectors. A given point with x and y coordinates is actually a vector between the origin and the point of the space with these x and y coordinates.
This is why I drew the lines between the origin and the points, to illustrate how the method works.
In your case, you are actually just looking for the angle of the vector between start and end.
const start = new Point(971.19, 264.87);
const end = new Point(971.19, 559.17);
const vector = end - start;
const angle = vector.angle;