They expect the pulverised rock to be spread by the blast and distributed on ocean currents, the CO2 is throughout the water column, it moves over concentration gradients, if one volume of water has 1g/L and another has 3g/L then CO2 will move from the 3g/L bit into 1g/L bit until they are in balance

I think they hope the pulverised rock will be spread so it works quicker, not having to wait for CO2 to balance