"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.