Note the “and Congress” line.
https://oig.justice.gov/about/
...The Office of the Inspector General (OIG) in the U.S. Department of Justice (DOJ) is a statutorily created independent entity...
...The Inspector General, who is appointed by the President subject to Senate confirmation, reports to the Attorney General and Congress...
By law, Inspectors General function under the general supervision of the agency head or deputy. The "independent" part comes from the fact that neither the agency head nor the deputy can prevent or prohibit an Inspector General from conducting an audit or investigation.
Have you ever worked in a federal agency with an IG and/or observed their audits? There are certain realities that govern what they do.