If I remember correctly, weren't there some height test balloons on that site a few years ago? Like maybe around 5 years +/-?
I'm not a geotechnical engineer and I don't live/work in FL, but I've seen developers build on all kinds of stuff...even over sink holes. Maybe the cost of building in the past wasn't where they wanted it to be, but I have a hard time believing that they COULDN'T build on it if they wanted to. With proper geotechnical exploration and a properly engineered foundation/pile system, you can build on just about anything.