That's true but only half the picture: every time one of these trees dies it is cut down it needs to be buried or otherwise prevented from releasing the CO2 back into the atmosphere. The tree you mentioned is described on Wikipedia as follows: "...This tree grows to about 6 m and has pinnate compound leaves and flowers with a boss of prominent reddish-purple stamens. It is not very drought-tolerant and the above-ground parts are short-lived but the roots regularly resprout..." So that's a significant amount of stuff to bury/dispose of. (The project at trees.org seems very interesting regardless, though.)