If you view the final episode of "Faces of America" in slow motion, you'll see that although Gates is talking about Meryl Streep's maternal Wilkinson line, the graphics are for her paternal Streep line.
Meryl used to think the Streeps were Sephardic Dutch Jews because she was told that there were people with that name today who are Sephardic Dutch Jews. Probably the least reliable indicator of one's ancestry I can think of.
As I pointed out in 2006, her Streeps were from Germany where the name was Streeb.
On the show Meryl said she was disappointed that her DNA showed such a European homogeny. She was probably let down when she learned that she is descended from ordinary German villagers rather than something more "ethnic." But I think Meryl knew the Jewish ancestry story was a myth before she appeared on the show. No sense showing a non-reaction.