Classic case of science “journalism” mixing up correlation and causation. The study only controlled for income in the broadest possible strokes, so in all likelihood, this is just one more study confirming that rich people live longer.
It’s possible that the green space lowers cortisol levels, thereby lessening the negative effective of aging, which would also account for the differences in Black vs. white participants (as Black people in America almost uniformly have higher cortisol levels simply because of the higher stress levels experienced by minorities in the US). More study (and rigor) is required to say for certain.
However, this headline is simply not supported by the study.