Earlier quoted context omitted.
But what happens to that carbon when the plastic decomposes, biodegrades, burns, is digested, or meets any other end? That once-below-ground carbon is now in the atmosphere, whether it takes a year or a decade to break down.
If the plastic is properly desposed of, this doesn't happen. When it gets into rivers and oceans is when this happens.
And recycling most common types of plastics actually releases more CO2 than creating new plastic products. There is no panacea one the carbon has been pumped out of the ground.