Lynn spent nine years in The League before getting the UCLA DC job. Look at the stat sheet first and the results are evident. Then look at the games and the eyeball test tells that story further. He's smart and organized. He's the son of a coach. He showed up and year one his defensive squad balled out in a conference where defense might as well have been optional.
He could be on a trajectory to go back to the NFL as a DC eventually, but he's now taking on the task of being Lincoln Riley's DC, so on paper it's a very good hire but the results are going to be predicated on how the head coach's mindset is about defense as a legitimate part of the game of football. Lynn did a season of the Chip Kelly Experience, so his sword is probably a bit sharper.
The only way USC is "serious" about anything is if their head coach acknowledges that side of the ball and takes a real interest in it as a football head coach should absolutely fucking do. Until he decides that's worthy of his time, I don't think Lincoln Riley is "serious", I think he's a jagoff.