It's fine if you want to draw some conceptual comparisons between biological and synthetic polymers, but it's 100% not true that "plastics" as defined as synthetic, organic polymers (I.e. acrylics, silicones, polyesters, polyurethanes, halogenated plastics, thermosets, thermoplastics et al.) are the same on a chemical basis as most biological polymers.

Like... where are you drawing the line? Are proteins a plastic? Is starch plastic? Is DNA plastic? RNA? Clearly not, by multiple definitions (bioavailability, reactivity, structure and function, persistence in the environment, etc.). Even biological compounds closer to synthetic polymers (cellulose, chitin, etc.) are definitively different, even if they do have longer persistence, lower reactivity, etc. And bioplastics (like what people mean when they say biodegradable plastics) are heat-modified biological polymers. They don't come out of a living thing that way; they are fundamentally altered from their previous form.

I guess I just... disagree that the distinction is "arbitrary semantics"?