They sort of obliquely refer to it, but omitted the attraction of clay to the surface of the grog/sand particles- the clay would rather stick to each particle than allow gravity or other force to separate them. this bond is “fluxed” by moisture while workable (a pug or other milling process acting just like a mason back-buttering stone/brick/tile) but once smeared all over each particle, the drying phase doesn’t weaken the bond and the particles of sand are each captured by clay, the clay wants to stick to itself and the sand so it cannot shrink- the loss of moisture instead creating tiny pores and a lattice of strong bonds like a huge brick cube. During firing, the clay is welded/“brazed” to each particle so you get enormous compressive strength.