This "social engineering" hack? is simply allowing a 3rd party to gain access to another persons account and "snoop" on their secured messages/calls.
Pls correct me if I'm reading this wrong.
However, I think there is a real possibility that the Signal code (of which the public appstore versions are NOT fully open-source) could be modified to save/transfer messages after they have been decrypted, basically circumventing the whole point of e2ee... which is why having control over the client code is essential.
I suggest either building Signal yourself, using only verified reproducible builds without any binary blobs, or switching to the Molly-FOSS fork.