Yes, of course a helmet doesn't prevent concussions, but they certainly reduce the quantity and magnitude of them.
The article is written like the writer is doing some bad interpretation of the comments made.
A helmet may not prevent the brain from rattling around, but it can and will make the intensity of the rattling around less.
That also doesn't mean that helmets couldn't be designed to better protect against concussions.
There have been studies done showing that some helmets reduce concussions better than others.