I strongly doubt, given the current State of the Art, that these DNA tests are truly that precise.
Regards,
If you go back several generations, you have ancestors from whom you have inherited no DNA at all. So lack of any Native American DNA would not conclusively prove that Liz Warren is lying. Even though we know that she is.
No American Indian ancestry myself, but I had cousins in the 19th century who married Cherokees and went with their families on the Trail of Tears to Indian Territory--while Liz Warren's white ancestors were prodding them along.