I think they meant that they can't say anything concrete about the causal relationship between personality change and change of life satisfaction, not about any kind of causal relationship at all. But you're still right. It's possible that personality change doesn't cause changes in life satisfaction and then we still can't do better than trying to make people richer.

On the other hand, the hypothesis that changes in life satisfaction follow changes in personality seems quite reasonable. They said "Fostering the conditions where personality growth occurs [...] may be a more effective way of improving national wellbeing than GDP growth." This seems to me more like raising that hypothesis rather than presenting a conclusion. (And if readers can't tell the difference then it's hardly the scientists' fault.)