I think it’s more a regional term (than race). I, too, have heard a ‘black’ woman from Oklahoma refer to her grandmother as … well, actually, ‘me-maw’. I’m from Boston and called my grandparents ‘gramms’ and ‘gramps’, so what do I know?
The context of this piece surrounds a dominant race (of white people) historically expunging an ethnic group (of Natives) and then attempting to claim their heritage for sociopolitical gain. Enter Elizabeth Warren claiming Native heritage, which is fine, but then pointedly using the terms ‘mammaw’ and ‘pappaw’ to prove a type of authentic nativity. That’s when I laughed, and laughed hard.
I don’t think anyone is saying that ‘white’ people or any people can’t claim Native heritage, but there is something very wrong with claiming heritage, culturally (of a persecuted people), for an advantage.