Besides, we know what the Preservers' motives were from their own words.
We might do well to remember what such publicly written testaments in Earth history generally tend to be - rather crude attempts at deception, self-boasting and whitewashing...
The scenario where the Preserves leave their Preservate behind and move on to new good deeds suffers rather significantly from the fact that the Preservate was a deathtrap. Indeed, the message was left on an instrument that was an integral part of the deathtrap, showing the Preserves were fully aware of the threat! Preservation of a culture also appears hopeless when the very survival of the culture depends on its perversion to serve the needs of the instrument left behind. (There'd be nothing left of the mobility of the communities, say.)
On the other hand, the scenario here differs from the one with the Briori or the Skagarrans in that there is no direct evidence of the evil masters being overthrown - or even of them spending any appreciable time with the abductees. Did the native Americans simply forget faster than the European Americans who for their part kept the memory of Briori and Skag cruelty and demise alive for a comparable length of time?