Or it would be if she hadn't claimed it on official documents.
White people often more or less fabricate a racial identity for ourselves other than white.
1. because most of us don't know why that's hugely problematic.
and
2. because white is boring. Particularly to people who don't understand no. 1.
Now, yeah, that's absurd. I'm not half-Asian, I'm half-Hungarian. But this is the sort of logical leap a lot of white people make.
Which is why a lot of white people didn't understand why Warren claiming native heritage was bad.