Radiators are plastic these days. My '97 Dodge Ram radiator is plastic. I figured the coolant would not harm the plastic. I normally clean engines with McKays Parts Dip, and I am sure to always remove plastic and rubber parts. But I did not think the coolant would harm it. What a surprise.
Radiators are a thermoset plastic, that is it is not melted with heat nor normally disolved in solvents and thus is resistant to solvents. Those Cox parts are probably styrene or some other thermoplastic.