The control group was drawn from Jews who were born in Palestine between 1911 and 1945. I think it's likely that this group would likely have been almost entirely Ashkenazi Jews, because there was substantial emigration of European Jews into Palestine prior to World War II.

See https://en.wikipedia.org/wiki/Aliyah.

(It's not relevant to your point, but although Sephardic Jews are not Ashkenazi Jews, not all non-Ashkenazi Jews are Sephardic. See https://en.wikipedia.org/wiki/Mizrahi_Jews.)