As others indicated, certain plastics contain oxygen molecules. The use of N2 in the system (not confirmed) may is a key to understanding the end results from the stack test.
Since certain plastics contain oxygen atoms, it can be released as O2 (oxygen) or recombine with nitrogen to form NOx - generic term for the mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide).
Thus, compounds containing oxygen atoms are not a problem - I agree with you there. However, oxygen (O2) potentially is due to its ability to cause combustion. But, as I indicated, the company has ran the machine without an incident thus far.
Cheers