Never heard of first nations but as somebody who grew up around Indian country I think we should approach this area with a little more scholarship and learn about what actually happened instead of jumping to conclusions.
Yeah, I thought it was a Canadian thing as well. Coming from an area that was heavily populated by Indians, it was a relatively large part of the curriculum back in my younger days. And I never heard that term in relation to Native Americans.
I'd be a bit reluctant to quickly judge someone's exposure to the various terminology around this topic. CGP Grey has a good video on the terminology as used in the US [0]. From the video: "Talking to people over the last [sigh] 5 years revealed a strong correlation: the closer a person had ever been to a reservation, the more likely they would be to use the word 'indian'. The farther, the more likely they would be to use the word 'Native American'." and also "At the time of writing [this video/2019], 'Indian' is used by Indians on Indian reservations to describe themselves"