Plastics leeching toxic chemicals/compounds into food and water, especially when being heated, is a known thing. It's especially bad for the endocrine system. The disruption to the hormonal balance is noticeably affecting people.
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.
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...