Your threat model seems to be an app whose published source code doesn’t match the published app, and whose published version uses a side channel not in the source code to leak messages in plaintext to a server. If that’s what we’re worried about then decentralization of the app’s main messaging channel makes no difference. The sneaky side channel could still be there in any app, centralised or decentralized.

That’s a theoretical worry to be mitigated through integrity checks on published open-source apps. The worry with Meta and WhatsApp is much more immediate: a known bad actor with a closed-source app, many domains they could use to leak keys or unencrypted messages, and a fawning relationship with the fascist and surveillance-hungry US Government. I’d still put significantly more trust in Signal even though it is centralised.