Solar intensity on Earth is 100 W/m^2? Where is that, the north pole? It starts out on top of the atmosphere at around 1300 watts per square meter and loses maybe half that or so leaving more like 800 watts per square meter or more.

Me thinks there was a conversion error. 1080W/m^2 is ~100.4W/ft^2.
Or, they could be averaging an average square meter of land across an entire year, including night and cloud cover?