[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.

It's true that if the CPU manufacturer is compromised that would compromise anything running on it, including attestation. But that's not really to do with Signal's implementation, and it is out of scope of the question.