To my understanding (though admittedly I'm not like a materials scientist or anything so take this with a grain of salt), "plastic" is a somewhat fuzzy term, the things it generally refers to are artificial polymers, but there are also a lot of naturally occurring polymers used in biology, and it's not like chemicals care if they're made by humans or not. One could call naturally occurring polymer materials plastic, but if you did that, you would have to say that, for example, things like wood and insect shells are made of plastic, which isn't really how people generally use that word. Naturally occurring ones are probably less likely to create "microplastic" type issues though because there has generally been enough time for stuff to evolve to break those materials down, whereas purely artificial ones are too recent for that.