So that particular map was done in a game engine called Unity 3d. It has a whole set of brushes/etc so you can deform terrain and texture it by just drawing with the brush. You simply import a heightmap for the base terrain with a couple clicks, then smooth it out with one of the brushes and you can draw trees/plants/textures with the brush. Extremely intuitive. Then you can throw buildings out and move/rotate them with the mouse.
But now I've switched to doing heightmaps in Blender because I need low poly terrains to run on mobiles and Unity's terrain gives you less control over polygon count.
In Blender I take a heightmap from photoshop and displace a plane to recreate the hills/valleys/etc. Then in Photoshop I select different shaded areas of gray and replace with my textures, then apply it to the mesh in blender, then import it into Unity to use in games.
I'll try to make a tutorial today if I have time, I just bit the bullet and learned the "blender" way yesterday and once I figured out how to do it, it's awesome.