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.
"Technically, silicone could be considered part of the rubber family. But, if you define plastics widely, as we do, silicone is something of a hybrid between a synthetic rubber and a synthetic plastic polymer. Silicone can be used to make malleable rubber-like items, hard resins, and spreadable fluids.
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.
