Nick, where did you find reference to an 8m length limit in the ADAC terms and conditions?
When I take a look in paragraph 2 entitled "Which vehicles are covered?" it says:
Vehicles with up to 9 seats (including the driver), up to 2.55m wide, up to 10m long, 3m high and no more than 3.5 tonnes GVW as well as motor homes up to 3.2m high and up to 7.5 tonnes GVW.
Not covered are scrap vehicles, vehicles in commercial use (excluding taxis) and vehicles with temporary licence plates or unregistered vehicles.