The scope of nixpkgs is nixpkgs. If every package's entire source code was being audited, then there is no such thing as a sustainable repository of software. There simply isn't enough people, or money for AI, to do it.
This would amount to auditing the vast majority of open source code found on the Internet. Not even OpenAI or Anthropic seem quite interested in doing that.
OpenAI and Anthropic both provide free credits for OSS.
Smaller models feasibly can review every commit IMO. Would not be cheap for sure, but also would probably be cheaper than a single fulltime employee.