Citations seem to indicate this is actually enough! This surprised me also: https://www.fs.usda.gov/ccrc/topics/global-carbon. Indicates 400GT are in from forests, vs 800GT in the atmosphere. Some various sources seem to indicate that around 50% of forests have been destroyed by humans already (by area, not necessarily by carbon?). So if we could instantly recover all forests, and nothing else changed, that alone would reduce the atmosphere CO2 to preindustrial levels (it has almost doubled). I think the misleading part about "millions of years worth of trees" is that a very small percentage of tree biomass probably becomes useful fossil fuel, and the majority is recycled many times.