If you look at the mechanical properties of of sand versus clay particles, the sand particle is much larger and inert. It does not bond or react with anything on its own. The large sand particles are rigid and keep their shape (unless you crush it to dust). Pure sand had huge voids in between the particles which water will readily flow through. The Ferry Morse article mentions coarse sand because it is irregular shapes. Beach sand has been ground smooth, basically into round particles so it packs tighter than the irregular shapes sand particles. Think of a big box filled with oblong footballs or rugby balls as opposed to round basketballs or soccer balls, depending on your sports of choice, but think of those oblong footballs or rugby balls as having five or six points, not two.
The clay particles are tiny compared to the sand particles and they are very active in that they bond (I think at the molecular level is the correct term but that soils mechanics course was several decades ago) to each other and they adsorb water, swelling and shrinking as they get wet or dry. They will also bond to the sand particles. And the clay particles contain a lot of ions and such that make them a rich source of nutrients for the plants. So mix in a buch of soft, squishy, sticky dots, maybe the size of peas or smaller to that box of oblong balls above. If you add a small amount of these small balls, some will stick to each other, some will stick to the oblong balls, but a lot of void space will remain so that water flows through it pretty easily and it can be stirred pretty easily. If you keep adding more of the small balls to the bin and stir it up, eventually you will add enough that all the voids between the oblong balls are filled. Water no longer flows freely between the voids because they are filled.
Other things get thrown into the mix too, on how hard it will set up, like mechanical mixing and heat. There are different kinds of brick and ways to make them. In those that use a lot of sand, the proportions of sand to clay are carefully controlled so the sand particles touch each other. That minimizes shrinkage. The mixture is mechanically agitated to get the clay particles physically close together so the bonds are stronger. (That's why you do not want to work wet clay soils. They really set up hard afterwards. I have experience with that.) They are heated to remove the water to create the stronger bonds that are formed at high temperatures. The clay particles shrink as they are baked, the water is driven off, and the strong bonds are formed. The clay shrinking is what makes the brick porous. Special clays with specific characteristics are used. I'm mentioning brick making to kind of demonstrate some of the mechanical effects of the different soils particles and what is working in your soils. It's not that adding sand will make your garden set up like a brick. It is the bonding of the clay particles that will do that, especially when agitated and heated.
I don't kow what the effect of adding sand to your soil will be. My soils mechanics courses were pointed at foundations for structures, not gardening. It depends on the size and shape of the particles and the characteristics of what is already there and your conditions. I suspect Ferry Morse knows a whole lot more about soils relative to gardening that I do. I don't think adding some course sand to a clayey soil will hurt but I also don't think it will help a whole lot unless you add significant amounts. I suspect that if you don't disturb the soil when it is wet, that the clay will shrink and dry around the sand particles without forming really strong bonds since you are not agitating it or baking it, and leave some pore space for water to flow a little better. I don't think working it when it is dry will hurt anything. Hopefully I am being clear on what is my opinion and what is my knowledge. Maybe this helps somebody make a bit of a better informed choice.
I do know I will not purposely add gravel to my soil. I've hurt my fingers too many times on the small rock I have in mine already.
