What gives this the added strength ?

I guess this aircraft is made out of High Density Plastic foam. The kind that is sold under the name of Kinney Foam and available in Refrigerant and A/c shops and Home decor shops. I have picked up some of it in 5mm and 6mm thickness but they are low density and warp easily.
Notice that when he bangs it on the runway, the tail section shudders and flexes on impact. Don't think Thermocol or Depron would behave that way. I could be wrong, though! Also note the carbon fibre rod across the wingspan.
Beautiful aircraft, love the scale undercarriage..........