That was his MO at LSU, he wasn't considered good enough to replace the other LB's but whenever he was in the game, he made plays.
Die-hard LSU fan here. The bolded part isn't really true. Cox started all 10 games last season, and even going into the season, was considered one of our best LB's due to the mass exodus of talent following the 2019 season (Queen, Phillips, Divinity...). Perhaps some fans were skeptical of Cox being a grad transfer from an FCS program, but he quickly put concerns to rest after a fantastic opening game against MS State (6 tackles, 4 solo, 3 QB hurries, 1 sack, 1 INT, 1 TD).
The reluctance to get Cox involved is solely a Cowboys quality, but I'm confident we'll see more of him in the future - he has a ton of talent.