The Wikipedia article linked literally says that they were not "unaware". It's right there in the second paragraph.

> The pilot then told the Haradas about the attack and the two agreed to help him. Nishikaichi and Yoshio Harada overcame a guard and escaped to destroy the plane and papers, then took Niihauans Benehakaka "Ben" Kanahele and his wife Kealoha "Ella" Kanahele prisoner.