I'm not a helmet "true believer" and don't wear one, because I don't consider my risk level high enough to call for one. But my risk level doesn't translate to anybody else's. But I do understand why helmet makers make few claims. They stand on the theories of testing agencies (Snell foundation, and ANSI - American National Standards Institute) which set the levels of protection and the testing procedures.

Helmets do reduce impact G-force in head strikes, and there is very good medical science that correlates G-force to TBI. Of course, this being biological, there will be variations human to human, but the overall ranges are well established.

Helmet makers are also careful not to make claims because that might be held against them when injuries occur. But realistically, reducing impact G-force, will reduce or mitigate injury. What may otherwise might be mitigated to no injury, what might cause a sever concussion, might be a milder concussion, and what may be fatal might become a major concussion. Notice, it's always might because there's no assurance, nor is there sure knowledge of what might have happened without a helmet.

We're used to these types of might help claims, because they're common in other applications. Statin drugs con't claim to prevent heart attacks, but millions of people are taking them on the theory that reducing cholesterol might reduce the likelihood of an attack.

Last thought. A helmet might offer protection against head injury, but the best protection is to not have a head strike in the first place. The best way to improve safety is to learn how to ride safely, which is not only about bike handling, but understanding traffic, and the other hazards of riding on roads.