"Rumors" is inaccurate. Rumors indicates that someone official said something unofficially that hinted at Greivous being a fleet commander.
What we've had, in actuality, is a bunch of players say that he should be a fleet commander and a bunch of other players say he should have a regular ship and a bunch of other players say he's too hard to get to be either.
That's not rumors, that's idle speculation. Which is all well and good, but let's not give it the legitimacy of being a rumor.