I don't necessasarily buy this rationale. In the mmaweekly article he talks about the grudge match being more important to him right now than a title shot. I've seen and read a lot of interviews from Rampage, and in all of those nothing has been more important to him than having a title and making as much money as possible (which comes from being champion). To me it reads as though he doesn't want to face Machida. Rampage was offered his choice, which means if he'd wanted to fight Machida, Machida would have been the other coach, and thus not fought until December.
To me the only possible challenger right now that they could pass as a pontential number 1 contender is Shogun. I'm sure he wouldn't pass on the chance to become the UFC champ like Rampage has done.