Maybe ratio of green to gray would be a better metric. Too much green or grey not good? Still, seems pretty tenuous until a country is beyond a certain level of development.
It's a very old, very small and well-preserved city, the poorer people tend to live in the villages (which are also very green, as it's rural and well-watered thanks to the mountains). Further, it doesn't have a large industrial economy.
I'm inclined to believe this article is the result of confirmation bias.
I'm currently looking for housing in charlotte, and tree cover tells me nothing about whether I want to live in a particular neighborhood. Like Ljubljana, trees are everywhere.