Presumably they mean that the CO2 captured in the wood of the tower can offset the manufacture of blades and other components at some point in the future. Not that reaching net zero in wind power is an important milestone or anything. From their technology page:

> The life-cycle emissions from modern wind power plants made of steel are about 4–7 grammes carbon dioxide per kWh. Building the tower in wood lowers the emissions from the wind power plant by approximately 30 percent per kWh.

That would put wind power some 50-100x below fossil fuels already. Additional improvements are always nice to have, but not really a big selling point.