He didn’t need to call his grandmother to get the facts — his mother was still alive when he wrote that book.
He makes it up as he goes.
Good point, although he might have been more comfortable contacting Grandma since he was probably closer to her anyway. At least until her "whiteness" became a political liablity. Also, Hawaii is easier to reach than Indonesia, Pakistan, or Timbuktu, or where ever she was at the time. :)