The point of E2EE is that only the people/systems that need access to the data are able to do so. If the message is encrypted on the user's device and then is only decrypted in the TEE where the data is needed in order to process the request, and only lives there ephemerally, then in what way is it not end-to-end encrypted?
Because anyone with access to the TEE also has access to the data. The owners can say they won't tamper with it, but those are promises, not guarantees.
That is where the attestation comes in to show that the environment is only running cryptographically verified versions of open source software that does not have the mechanisms to allow tampering.
The point of measured environments like the TEE is that you are able to make guarantees about all the software that is running in the environment (verified with the attestation). "If the software can modify data legitimately, it can be tampered with." - the software that makes up the SBOM for these environments do not expose administrator functions to access the decrypted data.
