Hi Santouche,
A simple question, with a very complicated answer that would contain more “it depends” than definitive responses.
Using a coir bed rug beneath your mattress will certainly help with air circulation under the mattress and assist with humidity control, but the farther away the coir bed rug is from your sleeping surface, the less of an impact upon overall temperature regulation it will have. You’d probably notice more of a difference by placing the coir bed rug between the wool and the foam of your mattress, but that would stiffen up your comfort quite a bit and probably not be quite so comfortable.
Placing a thin cotton mattress/topper on top of your mattress can help increase surface tension, which can assist with you not sinking in quite as much to your mattress, which can assist with sleeping cooler. This, of course, would also change the comfort of your mattress. Cotton will compact more than wool, is quite breathable, but is not nearly as good at humidity regulation as wool.
Wool is among the best temperature regulating materials but I would also keep in mind that it regulates temperature in both directions (it’s used in the desert and also in cold climates because of this) so while it may not feel “cool” … it generally doesn’t feel “hot” either and it also helps to regulate moisture (which can trap heat) and reduce the perception of temperature that comes from higher humidity levels (similar to how temperatures feel cooler on less humid days than they do on more humid days). Thicker layers of wool can feel a bit warmer than thinner layers.
As the article you quoted mentioned, items closest to your skin will have the most dramatic impact upon your sleeping microclimate. There is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that you may find interesting.
Phoenix