What people normally refer to as "Polygon" is their Matic sidechain, which is not a "layer 2" system. It's its own consensus system with its own stake who can steal funds transferred to it from the main Ethereum network. To be considered a L2 system it has to not require additional economic majority-trust assumptions like that.

See also extended explanation: https://ethereum.org/en/layer-2/ (which doesn't mention systems like payment channels since those are limited to just payments)

Polygon the company has developed a rollup based on zero knowledge proofs but afaik that hasn't been deployed with real money yet.