Why would it be impossible for Signal to decrypt and re-encrypt on their server while pretending to be "peer 2 peer encrypted" if we don't know what's running o

Why would it be impossible for Signal to decrypt and re-encrypt on their server while pretending to be "peer 2 peer encrypted" if we don't know what's running on their server? What's more: we don't even have the source of the client binary running on our phone. Also "which any nation-state can do" is an interesting angle. Let me put that differently: why would Signal NOT be a honey-pot operated by the CIA/NSA/other? Sounds almost too easy to not be true.