BIPOC stnads for Black AND Indigenous People of color.
It actually makes percect sense to me. The term specifies "Indigenous people of color." Therefore while you make a valid point that some Natives participated in the chattel slavery system, most who could not pass the Brown Paper Bag test faced the same discrimination and bigotry.
In my home state of Louisiana, over 75% of Indigenosu people of color became enslaved after losing miltary conflicts with Europeans. We mixed on the plantations and it is very hard to distinguish from the two.
BIPOC is not a term that is meant to replace "Black." It is simply a rallying cry, similary to the LBGTQ+ acronym used to politically note the similarities in our lived experiences so that we can make change.
Also, I think that genuine allies are a good thing, not a negative. But that's just my two cents