They say that at 3.7KW it gains range at 35km/h, and from solar it gains range at 12km/h. This means that they expect to get 1.23KW of power from their solar cells. Top theoretical efficiency possible for any solar cell is about 33% (Shockley-Queisser limit), most efficient you can buy today is 22%. So that means they expect 5.59KW of sun energy to fall on the car per square meter. A typical car is 4.5x2 meters. Let's assume they tile the whole damn thing with highest efficiency cells except the windshield (1x2m) They then have 7m^2, so they expect solar energy to provide 799W/m^2. And it is true, sunlight reaching earth is about 1KW/m^2 at the equator in noon, but that is in perfect conditions and just there, and when cell is perfectly aimed at the sun. The car will not tilt, and some days there are clouds or haze, and we do not all live at the equator. Also, in fact, not the entire car can be tiled with solar cells, and dirt exists, especially on the road.
So the numbers they promise are pretty much at the very limit of theoretical possibility in ideal conditions, and not even remotely likely to be hit in real life.