Yes, people smoke cigarettes, and let's assume for the sake of argument that it counts as "significantly harmful". Now imagine (hypothetically) that the same mechanism of thought also causes them to pursue lifestyles that grant them an overall 20% increase in healthy lifespan as compared to nonsmokers. In that scenario, the bias that causes smoking cigarettes is not "significantly harmful on average".

If anything like this were the case, I expect insurance companies would have picked up on it by now. It's possible to imagine a situation where the same bias balances out to not being harmful, but we have enough evidence to strongly suspect that doesn't describe the world we live in.