I did some very quick research and though I would share.
I was very skeptic of the idea that having stronger and stiffer muscles can prevent against concussions, yes they even mention stiffness (link 1).

To me, I was thinking you need more absorption of the energy and not resistance. Imagine your neck is made of steele, this would be bad news because the brain will have to absorb the shock.

This was puzzling given some data that people with stronger necks are less likely to experience a concussion (link 2)

Finally the answer came here (link 3) and here (link 4): Muscles do act as shock absorbers and the more neck muscle strength you have the more the neck are able to absorb some of the energy that would ultimately lead to a head acceleration.

Perhaps to you it was obvious, to me it took the link between strength and shock absorption to be convinced.

Otherwise neck strengthening is obviously good to protect against whiplash...which often is a component in such accidents...