Quote from the article: "Around 0.9 billion hectares (2.2 billion acres) of land worldwide would be suitable for reforestation, which could ultimately capture two thirds of human-made carbon emissions."
This is wrong. The reason is a bit technical. The issue here is that about half of the emissions going into the atmosphere get soaked up by natural sinks, e.g. the oceans. However if you take carbon dioxide out of the air the reverse happens: The oceans and other sinks re-release the carbon.
This needs to be taken into account, but hasn't happened here. So the effect is only about half the size of what is claimed.
Sources by some climate scientists explaining this: https://twitter.com/pepcanadell/status/1147066574299377664 https://twitter.com/hausfath/status/1147190442145898496
To be clear: There's nothing wrong with planting trees as one solution to the climate crisis. However this study was presented in a way that overestimated the effect massively. (Also I have some doubts that planting trees is "easy" given the political situation in the countries that have the largest potential.)