I don’t think it was ever exactly established what exactly was being referenced but look at the date. April 12, 2018. There was a lot going on context is important but Soon after Trump went to N. Korea also this:
NOW, THEREFORE, I, DONALD J. TRUMP, President of the United States of America, do hereby ask the people of the United States to observe the Days of Remembrance of Victims of the Holocaust, April 12 through April 19, 2018, and the solemn anniversary of the liberation of Nazi death camps, with appropriate study, prayers and commemoration, and to honor the memory of the victims of the Holocaust and Nazi persecution by internalizing the lessons of this atrocity so that it is never repeated.
https://trumpwhitehouse.archives.gov/presidential-actions/president-donald-j-trump-proclaims-april-12-april-19-2018-days-remembrance-victims-holocaust/