It's a fake rank that Optimus made to make Rodimus feel better.

[ in other words, Rodimus had been a brat and one of the only ways Optimus could think to calm the other down had been all these concessions... like the rank thing. while they both have equal power, Megatron has been the one running the show the last few months while Rodimus moped. ]

Drift... [ he's pinching at his nasal ridge now. yes, Megatron might have changed things for a few people. but Megatron is exasperated to hear it. people shouldn't be thankful of what he had turned them into, even if it's only briefly for Drift. ] That doesn't excuse everything else I have done or created. You don't thank someone that made you into a murderer.