Burning plastic sounds like an environmental hazard which is completely unaddressed in the article. At full scale, the communities near the launch site will never accept it.
Rocket fuels can have some pretty nasty stuff in them as well, of course...
The whole "don't burn plastic" only applies if you don't know what type the plastic is.
Pure HDPE is just H and C, basically like candle wax with a much higher molecular weight. Products of combustion are H2O and CO2.
Chlorinated and fluorinated plastics are the ones that produce lots of toxic emissions when burned.
Burning plastic incompletely would most likely produce more noxious stuff than Kerolox or Methalox engines. Maybe on par with existing Solid Rockets or Hypergolics.
No one is allowed to be near the launch path or within 2-3 miles of the launch.