It actually is closer to 5 years by my estimation, but that's pretty much it.

If you start with completely empty land, at first you will have increase in biomass, but after some time for new trees to grow you will need to have some other ones to die. The trees might keep exchanging CO2 to O2, but you will also get steady increase in emitted CO2 either through forest fires or microorganisms or animals eating and decomposing and literally breathing the CO2 back to atmosphere.

When things are in balance, forests do not actually remove any CO2 because any CO2 removed is balanced out by new CO2 emitted. Only planting a new forest on a new land binds some new CO2, and that only temporarily until the forest matures.

They offset that much co2 each year. It’s not a one time thing. Over the average tree’s lifetime, it will store 25-45 kg of co2 per year. So it’s not just a single year of co2. It would be decades to hundreds of years. Throw in renewables, fusion, nuclear, electric cars, co2 sequestration machines, etc, and it’s definitely a game changer.