Parent probably intended to say silicone, which is a plastic polymer. Silicone rings are commonly used even in stainless steel kettles or glass bottles for sealing the cap or lid.

Strictly speaking, "plastic" simply refers to the physical property of "plasticity". Though it's also used as a general term for "all polymers", which would therefore include silicone (polysiloxane) too. However people also use "plastic" to mean "organic polymers" specifically, thus drawing a distinction between plastics and silicone.

I get the sense that people on the internet believe silicone to be more safe (e.g. for sous vide cooking or baby products) for whatever reason. Maybe because it is more chemically stable or decomposes to sand or whatever. But really you can't draw many conclusions about any synthetic material unless there has been thorough research. A given polymer species can have very different properties depending on the additives used, and those can be the most dangerous to health. And the same material between different manufacturers could have different chemistry and health risks. It's a big complicated world...