I have read that Denny O'Neil came up with Optimus Prime, but Bob Budiansky came up with Megatron, and Shooter came up with the premise of Cybertron. Budiansky came up with "Fortress Maximus" and "Ultra Magnus" and those names based on O'Neil's original "Optimus Prime" latinate naming concept, Hasbro wanting literal names for most toys but grand names for major ones.
I've heard from a few sources that O'Neil had some input before he left for DC, but the basic premise of Cybertron, Optimus Prime, Prowl, possibly Megatron, and the names Ultra Magnus and Fortress Maximus are the only things that are consistently attributed to him.
I agree, plus it's not like Hasbro would even know that well because they put so many different steps in place between them and the content producers at Marvel proper.
Regardless of what Hasbro says, unless they have evidence, I'd lean toward Marvel's story, simply because it sounds like a case of getting lightning to strike twice (the original thirteen Joes were a rejected proposal by Larry Hama for Fury Force--and there's concrete evidence for this--who would've been fighting Hydra--you think it's coinkydink that Cobra and Hydra are so similar and both named after serpents?). It makes sense that they'd want to replicate the success, given that Joe was enormous and, at that time, they had no way to know TF would be even bigger.