You need to understand that the world "soil" is used very loosely.
A product such as Promix is not "soil" in the same sense that what is on the ground in you backyard is "soil".
Promix is a blend of different things but primarily(probably about 80%) it is peat moss. Peat moss is not what you have on the ground in your backyard... it is a very different thing.
Peat moss is a deposit of moss fibre that has been preserved over thousands of years. It grows in a peat bog, where the water has very little dissolved oxygen and thus things do not decompose. The moss grows and grows over many centuries and the layers of peat build up over time. This peat has been harvested and mixed with a few other ingredients to create "Promix".
The properties of peat moss verses what you dig out of your backyard are going to be so different that to equate them both as being "soil" is absolutely silly. Peat moss has a very high water retention rate, and is very much like a sponge. Essentially that is the function is serves in the super soil recipe. It sponges water up into your "soil" mix. Your "soil" in your back yard will not hold nearly as much water, because it is too dense and compacted. It will also not hold as much air because peat moss is fluffy and allows small air pockets to form.