Like all history what we know is not always what happened. I believe Chief Seattle was a great man and powerful speaker. The versions of Chief Seattle's speech are varied to suit different needs. The original speech is written by Henry Smith. Here are some interesting thoughts about it.
Rudolf Kaiser directed first scholarly fire when he saw the speech's widespread use among Green Party advocates in Germany.[6] Kaiser was struck by anachronisms in the speech, noting the blatant impossibility of Seattle referring to rotting carcasses of buffaloes that had been shot from marksmen on passing trains? Any speech by Seattle had to have preceded the mass slaughter of the Great Plains buffaloes by decades; the herds were exterminated in the 1870s and 1880s, and Seattle died in 1866....Smith's transcription was published in the Seattle Sunday Star on 27 October 1887...Smith calls his account "scraps from a diary," and says his reconstruction "is but a fragment of [Seattle's] speech, and lacks all the charm lent by the grace and earnestness of the sable old orator, and the occasion."...Smith never mentions the fact that he heard the speech entirely in translation. As others have pointed out, the speech was almost certainly intelligible to Smith only as translated from Seattle's Lushootseed (Puget Salish) dialect.[15] Smith may have understood the Chinook jargon (a trade language) used for intermediate translation that day; but reports are unclear about who, if anyone, made a translation into English, and whether that person had competency in Lushootseed as well as the jargon and English...Puget Sound historian David Buerge, who has researched the breadth of Smith's writings, notes that Smith "enjoyed telling romantic tales" and (perhaps carried away a bit by his subject) "imbued his work with a sense of murky twilight as he bore witness to the passing of the frontier." Buerge states that "Chief Seattle's speech [is] easily the best thing Smith ever 'wrote.'"..."Chief Seattle himself was a black-paint dancer who had only recently adopted Christianity at the time of the speech.[26] Thus, he knew the compelling power of the wintertime religious practice. He also knew the rage and resentment of leaders who did not share his Christianity and did not want to ally with Whites."
From Remembering Chief Seattle: Reversing Cultural Studies of a Vanishing Native American published in the American Indian Quaterly Summer 98 Vol. 22 Issue 3 pgs 280-304