You are absolutely correct.Pigment does not "sink" into the plaster but rather stays on the surface while the crystal forms around it. this is also a reason why the painting must be done with very "thin" paints otherwise the pigment will not adhere (crystals will not properly form around pigment particles)and will dust of when dry.
it is practiced often a "true secco" sometimes refered to as mezzo that is not really advertized because it is easily passed as buon fresco to frersco-unsofisticated person. i this technique painting is done when the wall is already dry or in process of drying - a thick layer of lime paint is applied to the wall and painting is done with pigments mixed into the lime paint and with galazes of thin paints - you can esaily spot that when you cant find a giornata (painting day) joints and it apppears that the painting is done in one section. it is possible because in order to keep painting all you need is to keep the wall moist by wetting. This is very commonly practiced technique, the result is a little chalky and colors are not as deep as buon fresco. also the surface is not as "tight" (strong) as with buon fresco and the life of this painting is not as long due to that.
PS about wiki - that part "pigment sinking" is not accurate, but as you pointed out "sinking" is not a well defind term in this case.
