It’s not just parks though, there are tons of options that don’t cost buildable land - the article called out sidewalks and medians, I’d highlight how well a goal of more trees fits with pedestrianized streets, or even traffic calming tactics like removing lanes. On top of that many cities have places you can’t build which are great places for greenways - bike paths on former rail easements, riparian boundaries along rivers and other waterways, even linear parks over shallow subway tunnels.