This is what auditors exist for - to bolster trust - they get to ask the difficult questions, look at the systems and processes in place, and assess the behavio

This is what auditors exist for - to bolster trust - they get to ask the difficult questions, look at the systems and processes in place, and assess the behaviour of a company in order to reassure external parties (often investors). They do often genuinely have access to data and code, even if it is perhaps slightly rarer for them to bring the skills to truly interrogate them. The reliability of an auditor is based on their perceived level of trust in the community. As an investor, you want to be assured that your investment is worthwhile, so you want to work with an auditor that is thorough and highly confidential. If an auditor can be proven to have intentionally misled in the past, or to have worked with shady agencies / outside parties, that would be a very black mark against them and future business, so it should, ideally, not be in their best interests. Note that this does not except the behaviour of humans in practice, where short term gains of a person might not be aligned with their parent organization (c.f. banking).