They're not running traffic directly on a road surface of styrofoam. A picture in the article shows a road embankment under construction, with a rebar mat on top of the geofoam blocks. This will have a concrete pad poured on it, which spreads the load of the road and traffic across the entire area of the embankment vs just point loading of the tires of the vehicles, so it can easily handle the weight.

There are different compressive strength levels available in expanded polystyrene. The stuff under our foundation for example has a compressive strength of 150 kPa. That's over 15 t/m² (>3000 pounds/sqft in medieval units).