Nonsense, a region that is forested has a proportion of carbon locked up in biomass indefinitely. If that region is deforested and no longer has any carbon locked up in biomass that is unambiguously a net source of carbon.
It's not locked up, it's continuously flowing in and out of that ecosystem. It's only locked up when it gets buried and can't return back to the atmosphere (except through technology). There's any number of reasons why a forest could get disrupted and die out besides intentional deforestation.
I hate to break it to you but we need all the time we can get. Also a rotting tree doesn't completely turn into gas, some of the carbon (I think 30%?) goes into the soil.