They could possibly do some good for lineman and since there is no real downside there is no harm in having them available but their actual preventive impact is extremely minimal. For what it’s worth Guardian themselves say that no helmet can prevent concussions, they have to say that for FDA purposes but it’s also true based on physiology.
It’s also worth remembering that the only way to diagnose a concussion is through a physical exam and mental acuity test. There is no imaging or any other test to definitively detect a concussion. So any claims of concussions going down from the NFL will have a massive bias. Players don’t want to report concussions for the future of their careers, the NFL doesn’t want them to appear to be a safe sport, and coaches don’t want them because it takes a player out of action.