>As the linked article admits, it only sequesters carbon if it is plant-based

At the very top: "It’s usually made from petroleum, which is better in a landfill than in a tank of gasoline"

"It" being plastic. The common non-plant-based kind.

And later: "If we're going to extract carbon from the ground at all, far better that it ends up in a soda bottle that will last 400 years than in the combustion chamber of your car."

It's good if there is more demand for non-plant-based plastic, because then we burn less fossil fuel.

>It is dumped in whichever way is cheapest and easiest

The most common way plastic is disposed of is in a landfill. The carbon is being sequestered underground, in other words.