When I put a spotlight between displacement ridges on a plane, the light doesn't start working under the upper (in this case bottom) most limit of the planes displacement. No matter what it won't effect volume until after the planes displacement boundary. It also obscures the light, the light source here in 1000m area is at 100,000 strength witha distance of only 500m.
For better or worse, spotlights in Terragen have an implicit minimum distance which depends on the aperture width. The "position" of the source is actually a focal point which is behind - or in this case above - the disc/plane that light is emitted from (the aperture), so in this case the aperture disc or plane is below the point you specified. To fix this you would need to change the position of the source or reduce the aperture width.