Well, not for polyethylene if done at high enough temperatures. And I doubt that the fungi will like plastic covered in toxic substances. But maybe there is a price advantage there if you don’t need to ship the plastic to a incinerator.

I didn’t read up on it in detail but apparently incineration has some large disadvantages, only about 22% of all plastic waste is incinerated. So fungi could be useful for the remaining 78% so it might be more useful than I expected.