Hi Alex,
They are there, you might just not be able to see them; the size of those dots are scaled, with the Brush Radius and also, if they are too far from the Plane (Sculptable Object), they will disappear.
So, if you move your Brush around, you'll probably notice (some of) them. You can also increase the Brush Radius and they will become more apparent. Or just zoom out, then the Brush will become relatively larger, compared to the Plane.
I'm not sure I follow the question exactly. You mean the blue dots of the brush? It's because Kent turned on tilling. Or maybe you mean why were there more dots when he changed the brush? With the yellow brush there were few dots than when he changed to the blue. That is because most likely Kent subdivided the plane but you didn't see it because there was an edit cut there. If you add a plane and go to sculpt mode and turn on tilling, you'll see there isn't even a single dot. But if you go into edit mode and subdivide the plane, now the mesh has more vertices and you'll get more tilling dots, subdivide even more and you'll get even more vertices and you'll get even more dots on the tilling in sculpt mode. So the dots visualization depends on the density of the mesh.