He didn’t implicate “good” Americans; he spoke of our leadership.
He implicated the leadership, and misled people to think the leaders ‘knew’ when in fact they were ‘told’ or ‘heard about’. There was no direct evidence.
The UK and the Americans did indeed request an answer from the Nazi government concerning alleged atrocities but the Nazis denied the allegations. International agencies did request tours of the detention camps and were granted ‘show’ tours but nothing could be confirmed.
Netanyahu stepped in it this time. He used the wrong words and ended up implicating leaders in allied governments and the people they represented. Reports of alleged atrocities occurring in detention camps did not come through a vacuum, they were not kept secret. They were simply impossible to confirm with clear direct evidence and one does not launch a military strike against a major power based on rumors.