There are two key quotes that you need to pay attention to in that document:
This engagement does not contemplate tests of accounting records or the performance of other procedures performed in an audit or attest engagement.
In addition, our services do not include a determination of compliance with laws and regulations in any jurisdiction. All inquiries made throughout the consulting process have been directed toward, and the data obtained from, the Client and personnel responsible for maintaining such information.
In other words, that document is not an audit. Just like previous "audits" published by Bitfinex connected companies, it’s an internal management report created for the management by an accounting firm. Such reports assume that everything they’re told by management is true: why would management lie to themselves? What would be the point?
If you’re an external entity on the other hand, eg a shareholder in the company say who wants an external third party to validate claims made by management, then you will only rely on a real audit made by a company that sends staff to check on those claims personally. This document is not such an audit & it’s authors are at pains to point that out.
Edit: The auditors did confirm bank balances with the holding banks, but did not perform an audit that would find any corresponding liabilities, nor that the claimed Trustee relationship between the company and the individual named on the account had any legal force. What this document shows is that people connected with the company had accounts with the claimed $Xmillion in them in total on the day in question & this was confirmed by the accountants. That’s it - it’s still not an audit.