They also mention that is not a showstopper and that it can potentially be countered with entertainment and exercise.
It also remains to be seen what the effect is like on humans. Might be negligible on a mars trip.
A more realistic approach to mitigating the issue is in my opinion to have rotating crews. They would spend most of their time in underground shelters, and only some minor fraction of their time doing surface work. Advances in medicine, specifically in regard to cancer, along with minimizing exposure time, is what I believe will make the risk low enough to be within acceptable bounds.
This is in large part why I think that Mars is continent upon offworld mining and construction industry, rather than the other way around.