Apparently it wasn't a "mistake". Apparently Volkswagen used special hardware and software to break the law.
Yesterday on PBS NewsHour the CEO of Volkswagen said the dishonesty was the fault of unknown rogue software engineers, and no managers knew about it. However, special hardware was designed into the system; that couldn't have happened without help from other people in the company, including hardware buyers.
See this article: Older VW diesels will need software and hardware fixes, Horn tells lawmakers.
The CEO seems to be lying deliberately. He says "software". Then later mentions "hardware".
That Auto News article was apparently written by someone who doesn't understand that, if hardware is required, the dishonesty must have been approved by Volkswagen management.