How to interpret 重地上，轻地下？ I think this is an oblique reference to 孙子兵法，but in a modern context it just means:

'the government only cares about what is above ground, what is beneath the ground is unimportant'

女：海绵城市能解决城市内涝的窘况吗？

男：长期以来，城市建设中存在“重地上，轻地下”的偏向。遇到强降雨时，最常见的办法就是通过管道“快速排出”。而海绵城市突破“以排为主”的传统管理理念，以建筑、道路、绿地等为载体，通过多种生态技术，构建合理的城市雨水开发利用系统。

What does the speaker mean with “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”。