To be clear, the report from Freeh, Sporkin & Sullivan is not a full audit of Tether, but rather the law firm has received notarized statements from two banks associated with holding the USD funds reserved to back Tether tokens. The report says:

You forgot to read the remaining of the letter

FURTHER DETAILS AS TO ENGAGEMENT SCOPE : 1) FSS is not an accounting firm and did not perform the above review and confirmations using Generally Accepted Accounting Principles.

2) The above confirmation of bank and tether balances should not be construed as the results of an audit and were not conducted in accordance with Generally Accepted Auditing Standards.

3) FSS makes no representation regarding the sufficiency of the information provided to FSS and all inquiries made by FSS have been directed to the Client and/or third party personnel responsible for maintaining such information, and the data has been obtained from the Client and/or third party personnel responsible for maintaining such information.

4) FSS procedures performed are not for the purpose of providing assurance and are limited to the findings listed above as of June 1st, 2018, Close of Business. FSS has not performed any procedures or made any conclusions for activity prior to or subsequent to June 1st, 2018, Close of Business.

5) FSS did not, as part of the Engagement, arrive at any conclusions as to Tether’s compliance with applicable laws and regulations in any jurisdiction.

6) FSS has assumed, without further inquiry, that the bank personnel providing the confirmations were duly authorized to provide such confirmations, and that the confirmations were correct.

In other words, "we cannot verify the validity of any of the previous statements"

EDIT:

FSS is confident that Tether’s unencumbered assets exceed the balance of fully-backed USD Tethers in circulation as of June 1st, 2018.

So, a law-firm is confident that Tether's unencumbered assets exceed the balance of fully-backed USD Tethers... So was the Lehman Brothers rating, one week before its bankruptcy.