Yes that seems to be the case, but I'm not sure that actually matters here. The timescales involved mean that there is plenty of time for carbon to move between

Yes that seems to be the case, but I'm not sure that actually matters here. The timescales involved mean that there is plenty of time for carbon to move between air and water. The only factor of much importance here would be the overall amount of CO2 removed. There might be some small time savings removing it directly from the ocean, but at 700 years out even 25 gigatons a year of carbon removal was insufficient. Given that kind of time scale, a few years earlier or later wouldn't matter.