Barr testified under oath the following:
"Special counsel Mueller stated three times to us in that meeting (March 5) in response to our questioning that he emphatically was NOT saying that but for the OLC opinion he would have found obstruction. He (Mueller) said that in the future, the facts of the case against a president might be such that a special counsel would recommend abandoning the OLC opinion but this is not such a case."
Summary - if we had real prosecutorial evidence that a sitting president was guilty of obstruction, we would throw out / step aside/ abandon the OLC precedent and proceed with charges anyway. The case against Donald Trump does not have the evidence basis to make that move.
Totally BS concept. Barr got the job by writing a nineteen page treatise stating POTUS above the law. Barr lied under oath multiple times to the House (earlier- see Crist questions) and to the Senate.