> Plastics are mostly carbon and hydrogen atoms

In general, this particular stuff is significantly different.

The article mentions sodium hexametaphosphate [0] and guanidinium sulfate [1], which have phosphorous and nitrogen respectively. Those are both common in fertilizers and are implicated in algal blooms.

[0] https://pubchem.ncbi.nlm.nih.gov/compound/Hexasodium-hexamet...

[1] https://pubchem.ncbi.nlm.nih.gov/compound/Guanidinium-sulpha...