Most writing uses "plastic" to mean any synthetic polymer, even non-organic ones like silicone. I don't think this is deliberately disingenuous or even ignorant

Most writing uses "plastic" to mean any synthetic polymer, even non-organic ones like silicone. I don't think this is deliberately disingenuous or even ignorant; it would simply be awkward and mostly irrelevant to use precise technical terms every time. Technically rubber is elastic, not plastic. But even "thermosetting plastics" aren't physically plastic anymore. Silicone is synthetic but not organic, while lots of organic polymers aren't synthetic. All the public really cares about is "shit that doesn't belong in the ocean".