Plenty of trees could be planted with $500 billion, but the timeframe to sequester the carbon the biosphere would be greatly extended. The reason that the author of the article discounts tree planting as a strategy for sequestration is that, as you may have noticed, trees release much of their carbon back into the biosphere in winter when they drop their leaves onto the ground. These leaves are then converted back into CO2 by the many fungi, bacteria, and detrivores on the forest floor.
As a result, there is more disruption caused by climate change. I think planting trees is an excellent idea, and that we should definitely do it, but it’s not an atmospheric carbon mitigation strategy.
If you are interested in this, look into carbon sequestration rates of switchgrass and elephant grass.