Most of the tribes practiced various forms of slavery. Everything from the "benign" killing off all males over a certain age, and making the women and young men slaves. to various tribes actively participating in Slave trade with the confederates, it doesn't get mentioned much but the union was actively fighting the natives during the early 1860s too. many tribes considered puberty the acceptable line, so it may not have been 9, but 11 doesn't come off as too much better, to me at least.
I would be willing to bet at least SOME of those tribes didn't draw too much distinction on age or use of those slaves. that's the problem with going with a holistic "Indigenous People's Day" with such a large sample size there is plenty of disgusting being overlooked.
its kinda like arguing that Columbus was the good guy because he didn't practice human sacrifice. both sides are doing some cherry picking to frame their argument in the most positive light they can.
and yes, a good number of holidays would have to go. but IPD was chosen out of some seeking of a higher standard, that itself doesn't pass.