No, this isn’t a mark-to-market loss, as the article clearly states—these are operating losses, literally more cash flowing out than flowing in. This is because

No, this isn’t a mark-to-market loss, as the article clearly states—these are operating losses, literally more cash flowing out than flowing in. This is because the Fed is trying to raise short-term interest rates, and to do this effectively it has to be willing to pay out its own interest target. These interest payments now exceed the interest it receives on its portfolio of government bonds and mortgage-backed securities. If this goes on for long enough then the Fed’s equity will become negative.