He's referring to the US government rounding up US citizens of Japanese descent and moving them to camps during WWII. It was a horrific thing the US government did. Unfortunately, because humans can be horrible, I would disagree it was one of the greatest crimes against humanity in history.
1945 was about halfway between now and when slavery was still widely accepted as the norm since time immemorial.
Statistically you would have to expect attitudes of what amounts to "humanity" to have only been about halfway from slavery to what there is now. Anything considered more modern would only have begun to exist to a much smaller extent at the time.
I think slavery was a bigger crime against humanity than the internment of Japanese, but regard them as both as among the most awful crimes the US has committed to humanity as a whole.
It wasn't meant to rank the worst things humans have ever done (outside of slavery no matter the country, that is almost always a distraction to justify awful things other's have done), but they are both among the most awful things the US has specifically done to the world.