PE burns much cleaner than coal, and probably paper too.
Burning plastic releases a slew of toxins into the air, including dioxins, furans, mercury and other emissions that threaten the health of people, animals and vegetation, according to multiple studies.
Unless they're somehow burning lots of chlorinated/fluorinated stuff (which is almost certainly not being used for much in the way of packaging, simply because those plastics are so much more expensive), that's just fearmongering. ...and mercury? WTF. That's where I stopped reading the article because it stopped making sense. Combustion certainly can't transmute elements, and mercury wouldn't be present unless they're burning something else...
...like coal, which does contain mercury as a significant impurity.