Agreed. All evidence points to them starting the war ages ago and being extremely aggressive. The new Guardian logs show that the same thing happened to them. Further, they were gifted linguists, figured out how to communicate with the Thargoids, and it made NO difference. So the Guardians had to respond with more aggressive measures and - I would assume - managed to drive off or contain the Thargoids. I can't imagine them fighting their final civil war AND the Thargoids at the same time.
And yes, most of the player base doesn't have the full background because they haven't read the novels - or bothered to check the wiki page that sums it up. We know from "Premonition" that the Club knows the Thargoids know we are sentient, that they know our language, and that they know we act as individuals and not as a hive collective. From "Out of Darkness" we actually see a rogue human scientist strike a deal with the Osresians, who build a base for him to conduct his research.
I swear, some of these players want to excuse everything the Thargoids are doing in game and blame humanity first. Some dismiss any and all data - from humanity or the Guardians - under "Victors write the History books." Well taken to that extreme, we might as well toss out ALL history as they are meaningless. Ugh. Yes, there is some bias, but that doesn't automatically refute all facts.
For my part, I'm surprised to learn that the Thargoids were NOT the automated constructs of the Guardians and were a separate, hostile race. After seeing how things went with the Guardian's peace overtures - and the fact that the Thargoids still HATE the Guardians 2 million years later - I'm all for using Mycoid 2.0, 3.0, etc.
The biggest eye-opening experience of all this for me is that I used to watch Sci-Fi movies where a group of people, totally oblivious to evidence indicating caution should be taken, would welcome aliens and get massacred (e.g. Independence day, War of the Worlds, etc.). I figured that was a lazy writing trope. Now I can see that no, there are really people like that. Wow.