THE QUESTION WAS NOT "IS YOUR GRANDFATHER JEWISH".
The question was, why won't you ADMIT that your MOTHER is jewish, and when did your family decide not to be jewish anymore.
IT's like being asked "when did you stop beating your wife".
His answer was perfect. If she had asked, was your grandfather who was imprisoned by the germans jewish? maybe he would have answered the question.
But as he explains in an article today, the way Fox worded the question, she was suggesting that his mother was a jew and therefore would have been expected to hate blacks and taught him racial slurs.
So he saw her attacking jews as being racists, and falsely accusing his mother of being a racist because she was a jew.
That might not have been the point of the question, but the full question certainly linked racial slurs, his mother, and her "jewishness". You have to go listen to her entire question while thinking about that, but when you do you'll see how it can be seen that way.