AFAIK, Signal doesn’t provide any way to prove its application were built from non-modified audited free open source code. Indeed there are evidences it behaves “a bit” differently.
So unless you’ve built application by yourself, you have no guarantee of it’s sequrity.
Apple's marketing buzzword silicon did nothing to shield Apple users against highly motivated and capable adversaries [1]. When the underlying OS is compromised (and with closed source OSes there is no way to ascertain) there is nothing that will save you.
