It would take some work, but if I were Buffy and 100% confident I was adopted, I would find a DNA sample from my parents (likely a hair on anything they owned, like a brush or clothing) or have one / both of my parents exhumed to take one tiny DNA samples from their body. With a DNA test, the result would be so definitive that anyone who had questioned my ancestry before would at that point shut the fuck up.
Yes she said that and also many other things in the past that don’t necessarily line up with that sentiment. Even if she does think she is indigenous regardless of her genetics, it seems like one would want to combat the perception that she is a liar. Doing the genetic test shows for sure you have nothing to hide, in my opinion.