Geometry shaders aren't needed to use cube-maps as a reflection texture like in your quote. Your fragment shader just needs to calculate a reflection ray and then use that to sample the cube-texture.
The parallax shader uses a cube-map as an ambient light source. To use a reflection map, you would want to reflect the eye-vector according to the surface normal, instead of just using the normal like the parallax code does here:
Code:
// Transform normal from tangent space to world space
vec3 normal = tsbMat * normalMap;
gl_FragColor.rgb = albedo * textureCube( tex7, normal ).rgb;
That other thread is about rendering *to* a cube map, which would require a few changes to Horde AFAIK.