Everything you've said as evidence is a supposed incongruity between description of different factions, and action in-game which can be explained in multiple different way from both sides of our argument, but you haven't explained why they would be lying which is really my central argument.
We know they've been manipulating a build up in forces to fight back, we know they have back up plans to flee the bubble if fighting back doesn't work and they've tried hard to keep that a secret. What I really want from you in an explanation of how a supposed lie about different factions helps their cause.
Because it creates and enemy where none exist. They've clearly convinced Bill Turner that it's true, and used that to get him to make the zero-wake FSD and presumably other things, who knows how many others have been told the same lie to get them to Co-operate against this organised, deadly force imminently heading out way.
When the club reveals the whole thing with the factions and civil war and stuff Bill Turner is in the room. He clearly already knows. The Club are expecting Salome and the heroes to be killed right there, but they need to continue the charade for Bill Turner to remain on-side. So they do this bond-style reveal, it's not for Salome and Co, who they expect to die right there, it's for Turner.
Rebecca's secrets are nothing about the supposed Thargoid civil war and all that junk, they are about the deaths of the Exodus crews and all those types of plans that were hidden.
The Club, by every admission in the novel, lie to manipulate. Having a powerful enemy to fight is historically a huge motivator, how much leverage have they gained from that?
Saying "we think Thargoids might still be out there, though we've not seen or heard from them for quite a while, we suspect they may be dormant" is not a good motivator. Saying "oh the Thargoids we met before were the peaceful ones, there's an even worse lot that's coming and we need you, important person, to help us prepare, but it's secret because, reasons, so don't tell anyone" is a classic manipulation trick.
Edit: wrote that while in a rush. Forgot to say: We have no real evidence that the Thargoids are hostile, at least not aggressively hostile towards us. At best, from actual field-research we can say they are typically aggressive in defence and otherwise they seem to pretty much ignore us.
This concept of fictionalised hostile Thargoids seems to be manufactured. Similarly the early "attacks" by Thargoids could well have been entirely staged. There's no direct evidence of Thargoid aggression, only reports of unprovoked attacks and strange exotic weapon damage. But we've had Thargoid tech for a long time, it's not inconceivable that someone made weapons that resemble Thargoid damage, maybe even staged attacked (assuming it was the federation, who seem to have orchestrated this whole thing from the start).
It seems more likely that the shadowy conspiracy is about pushing us into a war with the Thargoids as fast as possible. If you were someone who felt that humanities only means of surviving was to kill the Thargoids before they can return to full strength, it seems like this is exactly the way to go about it. And still, none of this requires or even suggests the versions of Thargoids presented by The Club.
