A rebellion isn't a rebellion until it's actually started, no one puts the lives of their people at risk for "planning stages" especially not if the soon to be rebel leader was presumed dead for a good amount of time. Also, who was Lor'themar supposed to turn to in the Horde? Vol'jin, presumed dead and his people equally as oppressed and persecuted? Baine, a world away and busy with ensuring that his own people don't get butchered inside Garrosh's city? Sylvanas and Gallywix only ever care about themselves and their own business. Lor'themar, Silvermoon, and the Sunreaver Onslaught were on their own up until 5.4. You'll see that once Vol'jin did start the rebellion, and once he cleaned up in Isle of Thunder, Lor'themar DOES go to the aid of his persecuted allies.