Many people may not realize it, but all sedimentary rock is the result of erosion and weathering. As the author pointed out sediment created by erosion tends to move down slope, creating a layers of sediment that are perpendicular to the force of gravity. If you ever travel through the Southwestern United States, this effect of erosion can be seen firsthand. This is why the layers in the red rocks of Sedona are all horizontal.
You may be asking yourself how this is true when there is sedimentary rock that is not horizontal. The truth is sedimentary rock may form diagonal layers, but only because some destructive force caused this to happen well after the sedimentary rock was formed. If you ever see that there is an outcropping of rock strata that is at an angle to the horizon, you will know that the seismic or volcanic event that caused this is younger in geologic time than the layers of rock strata that make up the outcropping.