I think you misread the Wikipedia page. Plasma gasification air emissions are cleaner with costs being somewhat higher than incineration. Plasma gasification breaks matter down at the molecular level versus combustion of incineration. With plasma gasification, you are paying more to end with a more inert end material and reduced hazardous process byproducts.

I interpret "can be cleaner than X and similar to that of Y" to mean "inferior to that of Y, but kinda close". Otherwise, it would be "cleaner than X and Y".