The documentation is wrong here.
A while during development of CMS 7.5 EPiServer.Web.ISiteDefinitionRepository did exist but it was removed later. So now the class EPiServer.Web.SiteDefinitionRepository is both the abstraction (all public methods are virtual so it is mockable) and default implementation in this case.
Ok. Thank you.
In the SDK EPiServer.Web.SiteDefinitionRepository implements EPiServer.Web.ISiteDefinitionRepository. But EPiServer.Web.ISiteDefinitionRepository is no where to be found? Where can I find it!?