The trees absorb CO2 from the air. Some of it is released when the charcoal is burned. the rest is sequestered in the ground.
Atmospheric CO2 ----> Tree------------------> Partial combustion ----> Atmospheric CO2
(Organic CO2 ) (Charcoal) +
Sequestered CO2
(Bio-char in the ground)
There may be some fossil CO2 to burn the wood in an oven, but if it's not 'cooked' there isn't. Natural gas and other mined sources of CO2 are the problem.