Actually being a skydive pilot myself, your stick and rudder skills improve enormously so I doubt the pilot was lacking in that regard. It's hands on flying all the way.
However I did find that others skills withered from lack of use. I sure wouldn't like to to find myself over the Channel at night in weather without recent practice and I actually held an Instrument rating with plenty of hours in actual IMC.
The possibility of spatial disorientation followed by in flight break up is what the report seems to imply. It's a common enough outcome, inadvertent entry into IMC on a VFR flight.