This one does not seem to fit the story as portrayed. Lets put aside the fact that "a better way to live" is likely be highly subjective and assume that they choose to live in tune with nature despite the ability to develop advanced technology.
As long as they remembered this fact, that they deliberated put aside technology, they would have behaved differently towards the humans. They wouldn't have needed the marine to show them how to blind the clearing machines, they would have already known. They surely would have at least debated pulling out the old tech when their soul tree was threatened even if they finally chose not to. And they likely would have approached the humans to try to teach them their ways and show them they were "better", or if they wanted to be isolated they would have had the knowledge to be more effective in driving the humans away which they didn't even seem to try to do until the humans attacked them.
Of course you could assume they forgot, but that seems implausible. A technological civilization that choose to abandon their technology should have little problem making sure that the message of deliberately eschewing technology and why would be passed down. If they didn't their descendants would be likely to start the cycle all over again a few generations. Also, their soul tree would be likely to hold this knowledge if no other method was passed (as I mentioned in a different post, the limits of the trees information storage and transmission are never made explicit, but it seems very likely it would have that particilar piece...)