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.)
‘Is Personality Fixed? Personality Changes as Much as “Variable” Economic Factors and More Strongly Predicts Changes to Life Satisfaction,’ published in Social Indicators Research (doi: 10.1007/s11205-012-0006-z)
[...] small positive personality changes may lead to greater increases in happiness than earning more money, marrying, or gaining employment.
[...]
We found that our personalities can and do change over time – something that was considered improbable until now – and that these personality changes are strongly related to changes in our wellbeing.
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Previous studies have shown that personality accounts for up to 35% of individual differences in life satisfaction, compared to just 4% for income, 4% for employment status and between 1% and 4% for marital status. However, because it was believed our personalities were fixed, policies to improve wellbeing have focused on these lower-impacting external factors.
[...]
“Fostering the conditions where personality growth occurs – such as through positive schooling, communities, and parenting - may be a more effective way of improving national wellbeing than GDP growth.”
[...]
Personality was measured using a well-validated personality questionnaire assessing five broad dimensions which cover the breadth of a person’s personality: openness-to-experiences, conscientiousness, extroversion, agreeableness and neuroticism. The researchers then looked at the extent to which personality changed and how these changes related to life satisfaction in comparison to external factors, such as changes to income, changes to employment and changes to marital status. They found that personality changes at least as much as these external factors and predicted about twice as much of changes to life satisfaction over the study period.
Link: manchester.ac.uk/aboutus/news/display/?id=8035