Fretless has said most of it, so I just can add a tiny bit more:

That's a fine example of how an urban legend originates. Seattle's speech was in 1854 and wasn't documented in a way we document speeches today. The first written version came more than 30 years later, in 1887, based upon memories of persons having heard that speech. If you know how reliably you can retell a speech 30 years later, you get an impression how reliable the 1887 text can be (the Cree citation above does not appear in this version of Seattle's speech).
In 1971, the speech has been rewritten once more to fit into an environmentalist conception of today. Perry's infamous 1971 version has e.g. this bit: I have seen a thousand rotting buffaloes on the prairie, left by the white man who shot them from a passing train. True as this may be (at later times), it cannot have been said by Seattle at the date of his speech. Perry has not even bothered to check the historical facts before writing his version of the legend.
The citation above is known as a Cree prophecy. No wonder it has been attributed to Seattle's speech as well by several authors not bothering to look into the first or later written versions of that speech.

Not checking facts, not bothering to read the sources, retelling tales to fit modern conceptions, that's how legends are made.
But I agree it is a nice legend. I love to read it. But I do not take it for true.

Wolfgang