I agree with you. In that render I raised the 'dry parts' a bit, and as that's a soft fractal, it also raised part of the wet area, producing the little humps. Not a very good way of handling the masking, but the result was nice.
The really far back is procedural trees, just a humpy rise and some dark greenish color, masked by distance shader, before that about 6 tree pops of 4 species and one grass pop inthe last render, taking the color of the ground. All displacements and colors are done after a base color node, so no compute terrain, easy in these kinds of renders.