If an auditor provides a clear plan in their preparation for an audit, clear descriptions of the criteria being used and then records their findings compared to those criteria, it'll be pretty clear that an unbiased, objective audit was performed. When anyone uses the 3rd party process, where there's little time spent on either of those aspects of an audit and every audit is viewed a done like a 3rd party audit, it's easy to see how objectivity is lost. "Proper planning prevents..."
We appear to have the popular "whataboutism" creeping in to these discussions. The plain fact is people can, and do, (internally) audit work they've done and have done it objectively and impartially and effectively, too.