It's not intended to be a carbon sink. It's essentially intended to be a more carbon efficient way of producing margarine without having to grow e.g. palm oil and destroy forests. They thought, instead of making plants do the work of turning water and CO2 into fats, let's just do it in the lab.

The basic science could work, although it's usually tough to beat "put seeds into ground and wait" on pure cost. However the fact that they compare this to butter makes me sceptical. Given how wasteful growing a whole cow is just to make some milk fat, it's easy to look efficient compared to that. They would compare themselves to sustainably produced margarine if they were honest.