If the client was open source, it could be verified by inspecting this source alone. To my understanding, the clients do real end to end encryption. This is the good part. They also have some functionality to re-encrypt the data or export the secret key to let new peers take part, or so i guess. This is how your web browser can also read them after you peer it up. Now there might or might not be a function in the client, where meta can request the private key or re-encryption. This is really hard to figure out without having the source code.