[ohthehugemanate]

Sure, but the question wasn't "does the NSA have access to data", it was "how do we know that information isn't stored."

The answer is that signal includes an industry-leading attestation process using CPU security features. If the CPU manufacturer is compromised that would compromise anything running on it, including attestation. But that's not a flaw in Signal's implementation, and it is out of scope of the question.