The plant grows pretty quickly as I understand it, and pulls CO2 from the atmosphere as it does so, using the carbon in its structure, and releasing the oxygen. This carbon will bond with oxygen from the air as it burns high in the atmosphere, so there's probably no net release beyond what is needed in the process of growing the plants, making the blocks, transporting them to altitude, and feeding everyone who gets the munchies afterwards, but it would "move" the CO2 higher up. I've no idea what the implications of that are.