It was certainly a driving factor. Britain was devastated and the population wanted nothing to do with foreign adventures. They lost India and then had no reason to bear the expense of maintaining standing armies in Palestine and Egypt to defend the road to India through the Suez canal. They pulled out, Israel was born.The idea that there wouldn't have been a Holocaust is ridiculous though. Hitler's justification for war was lebensraum. He wanted space for a Greater Germany. It always included genocide as a method to that end.
Very good point, that explains a lot. Genocide was on Hitler's mind for a long time.
But what we call the Holocaust took the form that it did because of specific circumstances. It resulted because Hitler controlled millions of Jews and couldn't use them as hostages.
Buchanan and his sources are concentrating on that smaller frame. In the big picture, I'd agree that what happened was the fruition of what Hitler had been talking about for a long time.