It isn't credible because the IRS IG has nothing to do with securities law and so they aren't the proper authorities. A simple read of their website shows it exclusively has to do with proper application of internal revenue code.
Fair enough. I don't put a lot of stock in the SEC anyway, a 'who watches the watchers' lack of ironclad faith in these regulatory agencies tends to afflict me. That said, all credit to them when they do enforce.