>End-to-end encryption is about protecting data at rest on the vendor's servers

No its literally in the name. It's about encryption between the end points of data communication. In most cases between two clients with the middle man server exchanging and hosting the encrypted data between them.

Its about protecting live data which in practice also results in at rest data protection because all data is encrypted which only the end points can decrypt. FDE for example only protects at rest data but does nothing to prevent live data extraction.