I'd have to agree to that. Flying down single track and fireroads at 20-30mph all day and coming off on a slippery bend is far more likely to happen that being hit by a car on the road and probably has more likelihood of doing your head or brain a serious injury.
(Although to be fair, in 4 years of MTBing in Scotland, the only injuries I got or saw were broken ribs, a finger, a toe and a testicle nearly severed by a handlebar - not me thank god - we ALL wore halmets).