The entirety of the signal "stack" depends on the SGX enclave. The fact that no one, in all time, has bothered to notice that the running code is different than the published code, is telling.
There's actually a newer SGX exploit, and related mitigation, that came to light at about the same time when they released their discovery protocol. Those mitigations were never backported to the base signal functionality. That no one audited and complained about this says quite a lot.
I've not looked at this code dump but perhaps the newer fixes finally made their way in. Or have been there all along.
It’s client apps who verify (via attestation) that the code inside an SGX enclave is what they expect it to be, and clients are open source.
> The entirety of the signal "stack" depends on the SGX enclave
Only private contact discovery depends on trusting SGX.