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.
We treat silicone as a plastic like any other, given that it has many plastic-like properties: flexibility, malleability, clarity, temperature resistance, water resistance."
It's a little misleading to just call silicone plastic with no disclaimers. That site is also called "life without plastic", so I'm not sure it's the most neutral source.