Oresrians and Klaxians are official lore, or else Frontier wouldn't have approved of their mention in the novel Premonition. The Club is also lore for that same reason.
The problem is that game design hasn't proceeded in such a way to show us how this lore manifests itself in the game. We have Thargoids, yes- but no way to know which is which. The Club, too, can only be assumed to be pulling strings from behind the shadows, which has the added bonus (for Frontier) of shifting that particular storytelling burden onto us, the players.
It is not a matter of if the Oresrians and Klaxians are official lore from Frontier or not
But if the Characters in the official fiction talking about Oresrians and Klaxians are actually factually correct about them in the in game universe.
Characters can believe it to be true but it is actually wrong, in universe.
This belief in the Oresrians Klaxians Civil war has been a driving force behind the actions of several powerful factions in the game lore, but if this belief in something that turns out to be wrong, could explain why all the preparations seem to have ended in nought.
I mean the Club has been an absolute failure to date.
The Dynasty Project a failure as otherwise there would be colonies there already, once the Thargoids arrived it was too late
It was also a plan based on Pre FSD technology which out moded it.
The Push the Superpowers into a military build up yielded increased numbers of Capital Ships useless against the Shut down fields
The Wake Suppression tech to allow the escape seems to have been pointless as the Thargoids don't seem to be have bothered humanity when we are not near Barnacles so there was no need to conceal the retreat to the worlds that were never colonised as part of the dynasty project.
Maybe if you consider the Club was behind the free Sidewinders and CQC and thus succeeded in creating a Combat force out of the independent pilots of the Pilots Federation to combat the Thargoids, that might be their only win, but that relied on the exploitation of Guardian Tech, which was a result of Halsey's Visions (being made into a YouTube video...) which is something so unreliable it can have been part of the Clubs plan so just a happy coincidence.
All this seems to point to the Oresrians Klaxians Civil war being Canon Misinformation fed to Humanity by the Thargoids in order to make sure humanity was ill prepared for their return, with Humanities elite ready to flee, and fleets of ships capital ships vulnerable to
A possible Galaxy spanning civilization millions of years old, is surely able to pull off a Caprica 6 level deception to undermine humanity's defence preparations, by exploiting the fact that Humanity is not a Hive mind but Trillions of individuals that can be manipulated
In the Novels we are only exposed to the idea of the Oresrians Klaxians Civil war by Characters telling other characters of it, second hand
As readers, from our greater vantage point, we still never see an explicitly Oresrian or explicitly Klaxian or a Thargoid kill another Thargoid
We only get reported information.
I think that is a deliberate choice.