Old guy, let's break this down into Basic ....  

Mazars said "They can no longer stand behind the financial statements they put out for Trump in the last 10 years" ...

No matter what else is in the letter, the firm is clearly saying "Do Not rely on the statements we put it..."

Mazars said it had based its conclusion on a January filing by the New York attorney general, its own investigation and information from internal and external sources.

"While we have not concluded that the various financial statements, as a whole, contain material discrepancies, based upon the totality of the circumstances we believe our advice to you to no longer rely upon those financial statements is appropriate," Mazars said in the letter addressed to the chief legal officer at the Trump Organization, Alan Garten.

You re getting hung up on the phrase "While we have not concluded" - It's an ongoing investigation, but they HAVE concluded that you may NO LONGER RELY ON THOSE STATEMENTS" - So that means, at one point, it was okay to rely on them. Now, they are saying, BASED on their own internal investigation, you should NO LONGER rely on them.

If they were confident they were accurate, then they would have no need to put out that statment. I don't understand how you can ignore the body of the entire memo and hyperfocus on a phrase that is not definitive. The phrase "We have not concluded" ... that means it's an indefinite...they are still getting data to REACH A CONCLUSION. There is NO CONCLUSION reached YET ...

When they are ready to "Conclude", they can say "We have CONCLUDED that the financial statements are accurate" ... or "are inaccurate" ... 

So there is enough doubt in their mind, as to the accuracy, that they should not be relied upon.

This is basic Logic Old guy .. it's not "let's spin this so that it fits my narrative"