It's even worse than that. Tether fired that auditor (Friedman LLC) because they were looking into it in a bit more detail.
From the very same article: "Given the excruciatingly detailed procedures Friedman was undertaking for the relatively simple balance sheet of Tether, it became clear that an audit would be unattainable in a reasonable time frame."
Even Friedman LLP says in the internal memo “Do not rely on this report”.
It's been almost 4 years since that and no independent firm has ever audited Tether.