How to interpret 重地上，轻地下？
To me, as a native speaker, my intuition is that it is not something from old Chinese. Not all those sentences or phrases that use very few words come from old Chinese.

In this case, they just want to briefly summarize the current preference on urban construtation that people focus more on things above the ground rather than below the ground.

Actually, it is quite common to say “重A，轻B” in formal situations. It is also very common that people summarize some key characteristics in a brief way, making it looks like old Chinese. You can find a lot of them in some political files or slogans, such as “兴国之要，兴国之魂，立国之本，立党之本”。