If you extrapolate the charts, it looks like if they simply waited one more year the mental health benefits would be the same.
Moving to a less green area appears to be accompanied by an immediate drop in mental health, followed by a steady increase. The only difference from those charts when moving to the rural areas is that there is no immediate drop, which appears to account for all the difference. And the mental health benefits of rural areas appears to plateau, so it's completely possible that the move to the less green area will catch up in the mental health benefit aspect within the year.
Moving to a less green area appears to be accompanied by an immediate drop in mental health, followed by a steady increase. The only difference from those charts when moving to the rural areas is that there is no immediate drop, which appears to account for all the difference. And the mental health benefits of rural areas appears to plateau, so it's completely possible that the move to the less green area will catch up in the mental health benefit aspect within the year.