Anyone being able to review code is NOT the same thing as an audit. An audit is a more formalized process where there's a more defined process and some form of assurance of quality is provided by the group. A formalized audit should cover all, or at the least "critical areas" of the code. An audit also might entail more than just the code, but who has access to it, what the commit procedures are, etc.
What you're describing is more ad-hoc. Individuals going in and making sure there's no glaring errors or design flaws, or even more subtle things if they're so inclined. I'm sure that's happened, but how do we know to what extent?
Audits are more about providing more formalized public assurance of code quality. Both the ad-hoc code inspections, and audits are useful, and mutually beneficial. But they aren't the same thing.