Unlike Brabus ( which indeed is a tuner ) Ruf is a certified manufacturer according to a rule in German law ( another one is Alpina ), both Ruf and Alpina get an unmarked chassis ( no chassis number attached ) and parts which they use as a base to build their cars and then attach their own chassis number to it.
This is only possible ofcourse due to the close relationship and respect both Porsche and BMW have for respectively Ruf and Alpina and vice versa.
Both work together in developing parts and share knowledge ( BMW for example uses a transmission on their own cars developed by Alpina ).
So although it may appear both these companies are tuners they're not as both are manufacturers in their own right and can therefore decide to give their licence to a game without consulting or asking permission for it by the "parent" company.
A Brabus is still a Mercedes-Benz ( a Mercedes chassis number ) even if it's badged a Brabus, a Ruf or Alpina however aren't a Porsche or BMW due to the reasons mentioned above.
Although technically they might be perceived as tuner firms by those unaware they strictly and legally aren't.