it's also true to some extent. now, we can't really know what was going on before the europeans showed up, but these were not super warlike people. they just didn't have the technology to do that.
now, down south in mexico land, yeah, bunch of genocidal cannibal maniacs. the carib were eating their way through the arawak when columbus arrived (exaggeration). but north america? eh. i'm sure there was some uncouth stuff happening, and the noble savage is pretty well a myth, but equating isolated violence with the mass exterminations that the europeans wrought on north america is just silly.
Why is the Noble Savage a myth? Rousseau glorified it. Franklin milked it for all it was worth and that image in France is huge as it directly leads to the birth of the United States. This doesn't nullify the power behind the stories.
The Native Americans were not technologically gifted, this is true. Spiritually, it's another story. Their power is in their bodies and their skill. In Texas, we say it's not the arrow, but the Indian, you have to worry about. They have great spiritual power, live in harmony with nature and the land. They are men who live life, not sitting behind desks counting money. Put them in jail, they die. They walk a spiritual path to life rather than being trapped by mere flesh and blood.
There are territorial wars, and the rise and fall of nations. You have very peaceful tribes, some fairly warlike tribes, and most tribes have some skill at raiding and killing their enemies if only at skirmishers levels. You have the literary ideals of Chingashgook and Magwa, and the real life examples of Sitting Bull, Geronimo and Tecumseh. Good men, and bad men. And men who are hard to explain, like the Prophet, Tenskawatakawa, with his Zero Year curse. There's a lot of juice in these stories.
But for the Railroads to come, and the great Western Expansion to happen, they had to go. Violently. The Yankee army, whose origin myth is rising up to liberate the Black Man - because all men are created equal - had no problem being the hammer hitting the anvil and pounding the Red Man out of existence except as museum exhibits. Which tells you all you need to know about Yankee propoganda.