I have some anecdotal evidence of my own, as a white dude. My grandfather was a professor and quite a radical one back in the day, eventually moved out to Montana to assist in the founding of the English dept of a new community college on a reservation. Naturally he got involved with radical politics out there and became associated with the AIM or the American Indian Movement (check out the second battle of Wounded Knee). The reason why we say Indian in my family are his and my father's experiences with them (in the AIM & other groups) and the regular people on the reservation they lived on for years.

I guess it's anecdotal as well, that's why I mentioned AIM, it's a good starting point if you want more 'objective' 'evidence' for the opinions of groups of people who very much still exist and have no problem speaking for themselves. Anecdotal evidence is particularly bad when you're talking about rareish, one-off experiences where some random variable is asserted to be a constant or at least correlated (a frontiersmen is eating an apple walking down a path and a mountain lion attacks him, after he escapes he tells his friends that mountain lions are probably attracted to the smell of apples). If people from (one of) the groups being discussed are telling you that in their day to day lives something is common knowledge, that's a bit more than just spurious anecdotes.

No group has 100% unanimity on anything, of course, but it's obviously at the very least a valid and widely help opinion. I can see no reason to accept the musings of someone who has likely never even met an Indian or been to the US over literally every Indian I have ever met, my family's direct experience, and numerous political movements that are only a google search away.