You need to convince the rest of the market that the company is fraudulent - and that is a very expensive and complex process.
...but it does illustrate the value of short contracts in finance. They incentivize investors investigating and raising the alarms for the rest of the market. This company was particularly aggressive in defending themselves publicly, so it didn't happen until their market valuation was high enough to make it worthwhile for the big shorts to come in and crush it.
The auditors will do interviews with people, but the company also (somewhat) picks who will get interviewed.
Remember that auditors are paid for by the company they are auditing. They are motivated to do the minimum required to pass the given regulation. They aren't there for true oversight, they are there to make sure all the checkboxes are checked.
The fact that something this obviously fraudulent dragged on for over a decade is nuts.