The "burning" is in a sealed environment using "temperature-gradient thermolysis"; this is a type of thermal degradation that doesn't introduce oxygen but uses heat directly. Similar to how a cracking tower works in the conversion of crude -> useful hydrocarbons. Really the burning in the article was used as a metaphor.

Biodegradability will depend a lot on the final product that is being made, this process makes a precursor chemical phase.

There will almost certainly be incomplete conversion, there will have to be a purification step before it is fed into the next process. The real questions here are:

What percentage contaminant is present?
How hard (costly) is it to purify?
What is the waste product and can it be fed back into the cycle to be re-processed?
What temperature does the thermolysis occur at? (higher = costlier)
What is the temperature gradient? (bigger difference = costlier)
Does running this at higher/lower pressure increase efficiency?
How does this product compare to virgin material from crude oil -> the same precursor?

There are a lot more questions I could come up with, but the fact that the process is relatively simple and can take multiple types of plastic is what I found really exciting.