In my humble opinion, the MSU job could be nicer for Rossi, he most likely will have more dollars for recruiting, hiring assistants, and training. It may be a good step for his career.
However, in a business sense, none of that has anything to do with it not being a lateral move. What does is what he was hired to do. He will be the DC at a B1G school, he will plan, implement and call plays for the defense. He will manage defensive coaches. And he will report to the head coach. All exactly the same as his job at Minnesota. A promotion would be something like adding Assistant Head Coach duties to his job.