I think it was Farage's intention to outrage the members so that he would cut the ground from anyone who wanted talks to go slowly and in a friendly way. He doesn't want any backsliding.
That, and he wanted to say what the then German Chancellor said eighty-odd years ago 'They laughed at me, and they're not laughing now’. And I think he wanted to savour saying it precisely because Hitler said it. That isn't fanciful. He "marched through a quiet Sussex village very late at night shouting Hitler Youth songs..." [ Letter from a teacher when Farage was at Dulwich College ]