Swap is a function, a feature. A DEx is a service, a platform. A swap isn't a function or feature that is exclusively provided by decentralized services or exchanges. You can do swaps even on centralized exchanges. You can also do swaps on services falsely claiming or marketed to be decentralized.

I guess this is the more important point to consider. Just because it is a swap platform doesn't automatically mean it is already a decentralized exchange. And just because it promotes itself as a decentralized exchange doesn't necessarily mean it is truly decentralized. A mere label doesn't mean anything.