It's explained in the article. They have combined satellite imagery with observations from underneath the canopy, so they had both the area of forest and tree

It's explained in the article. They have combined satellite imagery with observations from underneath the canopy, so they had both the area of forest and tree density in the particular forest. > The scientists were able to reach these estimates by merging together two separate mechanisms for sampling trees — satellite observations and ground-based ecological work. The former gives an overall view of where forests do and don’t exist on the Earth’s surface. But the latter goes underneath the canopy to determine how many trees exist in a given area in a given type of forest. Thus, the study amplified satellite views with no less than 429,775 separate measurements of the density of trees at different locations around the globe.