That's a good point, but it is relevant that Elon Musk isn't just a "scientist" or "expert" but a businessman positioning himself to sell a product.

I'm willing to give him the benefit of the doubt on this one and wait for more details, but at the same time I concede that it's not unreasonable to be wary of claims that the radiation problem "isn't a big deal" from someone planning to sell tickets to Mars. Not everyone has the same notion of what an acceptable risk is.