It seems that the followers of the absolute do not know that they have tadpoles. I guess Minthara would be surprised if someone told her. Could be an interesting dialogue.
This is an interesting point. You run into people from the ship who know you have a brain slug and you run into followers of the Absolute, who know you as a true soul. From the first, a relatively recent excursion by a spelljammer where the crew had no qualms about making you watch them drop a worm in your eye; and the second a relatively new (for a religion) cult that is (if you believe the dead mind flayer) a group of the same type of folks on the spelljammer but ...didnt? let you know they put slugs in your brains?
Its good that I can't figure out their plotline - it means its not simple - but if you have, at the surface:
1. A roaming, kidnapping, spelljammer squid group implanting dormant slugs blatantly
2. A subterranean squid group implanting dormant slugly slyly
3. A hag that said the slugs were protected by netherese magic
4. The netherese were artificers and have since gone extinct (though, I suppose Gale could be one...hmmm)
We could likely assume that the squids are sourcing dormant slugs from the same rare source - some kind of netherese artifact.
But! The people on the ship seemed to be between school teacher thralls and ones scheduled for exaltation. There didn't seem to be a middle class like these "True Souls". You either were a slave or you knew you had it but thats ok, youre about to be a squid. The class structure is weird because Minthara, if you lie to her about Halsin and say youre on a mission to get him, says "I have a new mission for you, kill the grove". You ask why and she says because it shows our power in the region the absolute has claimed. So wait, you can just give me new orders? Can I just give you new orders right now then, too? Is that how True Souls work?
If you tell her to get out of your mind when she mind reads for the grove you can bluff and say youre the chosen - a failure has her say "good, then killing you will show my superiority".
So, it does seem they don't know its a slug. Minthara just says she can feel the Absolute moving around in her and recognizes it in you. It doesn't seem to have any issues with brain slug carriers murdering each other, even though it means the ejection of the DuraSlug™ into whatever environment the host died in. Plus, that would let the other True Soul see that and feel a connection to it - potentially revealing their own filthy secret.
Im not saying they mean to have a bulletproof plot right now considering all the little pieces of information you can gather and then apply logic to, but I think that the importance of the DuraSlug™ and the loose control over the True Souls and their extreme aggression. And that odd scene at the start where the True Soul dwarf died outside the Owlbear cave and they were searching for ship people - and were KOS to them apparently as they tried to slaughter you the second they found out. So, why are you all to be eradicated? Youre "True Souls" who know the truth and didn't get exalted? Those ships are there to make one of two things? The underground place goes through the extra effort of subversion to create armies? Thats a lot of noise to be making right now for a small army with a new god bent on domination and subjugation and the subversion or destruction of other beliefs.
Overall, I remain confused by their tactics.