Regardless of any centralized server, the open sourced client code ensures encryption prior to being sent to the server.
E2E encryption === end to end encryption
The server code shouldn't matter, because it can't read the message to begin with.
It depends how it is signed and how the secret key is generated. WhatsApp also have e2e encryption.
Are you suggesting those details are not verifiable from the client source code?
WhatsApp also has a built in function to send the last 5 messages to Facebook moderators...