This rebuttal article puts it succinctly in that it’s inaccurate to call concrete a sink because on the whole it contributes more than it takes out. Additionally, the 50% number quoted doesn’t include fossil fuel emissions used to make the concrete (roughly half the total emissions).
Also interesting that the process requires certain conditions:
Shah also said that “cement carbonation requires very specific conditions” including humidity of between 40 and 80 per cent and open-air conditions.
“Submerged or buried concrete or concrete will not undergo carbonation,” he said, adding that “concrete carbonation happens at an extremely slow rate: an average of one to two millimetres per year.”
… implying that this might not occur in regions lacking this humidity and not at all if buried in a landfill.