If you just have one big control, this still does not tell about variance inside the control. You can subdivide the control in N groups, but I think you quickly

If you just have one big control, this still does not tell about variance inside the control. You can subdivide the control in N groups, but I think you quickly increase the noise to signal ratio. A control group split into two is a good compromise, and intuitive to reason about, like the author points.