There is nothing wrong with their statement. If the hub dynamo has its own voltage limiter, then the E-Werk can supply no higher voltage than 7 Volts. If the hub dynamo has no internal limitation, then the E-Werk can supply the full output range.

SON hub dynamos have no internal limiter. It is in the backlight cable supplied by Schmidt, which you should use with limiting function behind the light switch if the headlight has no limiter at all..