I think it's just painted, no bed liner applied. My Silverado looked the same before I put a bed liner in it. My guess is steel, though no way to tell, or to know if that's a production intent prototype or not. I remember seeing the Slate Truck in person and being told the one they had for demo...
For sure, no way they just happened to get a tip about the day, time, location etc. In the video too, the UEV pickup pauses for extended periods to let the guy record too