The triplanar is not related to landscape HEIGHTS, but the angle of the surface, allowing you to texture "flat ground" differently from "steep ground".
Triplanar works by taking a cubic projection, and projecting a different texture from above, from the left, and from in front. "From above" will refer to flat areas of the landscape, and "from the left" and "from in front" will refer to more vertical areas (both of those should be the same texture for a landscape, since both represent "more vertical", just facing in different directions which wouldn't really have a difference in most landscapes).
If you want to start changing textures via height, you'd have to look at building up some nodes that also include the CoronaDistance map, using a flat plane as the "distance from" object.