The quantity of plastic thrown away is small enough that burning it would be a very small proportion of carbon emissions. This becomes clear when you notice tha

The quantity of plastic thrown away is small enough that burning it would be a very small proportion of carbon emissions. This becomes clear when you notice that plastic production is a very small part of oil consumption. The world produces 260 Mt of plastic per year but burns 7000 Mt of coal and 3000 Mt of natural gas in that same year. So even if we burn all the plastic and ignore the emissions it’s not large.