@ algae is a major CO2 sink
The plan is to grow marine phytoplankton (microscopic algae) in large raceway ponds, not harvest natural marine algae. In theory this should be a zero sum game where CO2 taken up during growth is released during burning of the hydrocarbons. The protein the algae produce can be used for livestock feed. The only net loss in the system would be the inorganic nutrients used to grow the algae, such as nitrate. But the deep oceans are full of inorganic nutrients.
No rewriting of science here, just common sense.