Thats probably (!) right...

If you smash your own car up all by yourself on the Ring (Regardless of legal "Public toll road" status) and then try to claim, they would obviously not want to pay you a penny.

If it is a third party claiming against you, it could be more in your favour.

Personally i accept that if i bin my own car, i wont get a penny back from the insurers and could potentially have a very large armco / emergency service / helicopter / track closure. And i try to drive with that in mind, mainly putting in laps at quiet times.
Doesnt stop accidents happening though
I cant imagine the complications of being involved in a multi car shunt...

I expect youve already read this but its worth a look if you havent:
http://www.nurburgring.org.uk/insurance.html