It's already well established that Thunder Warriors are in many ways prototype Astartes. That doesn't mean they have gene-seed.
You are getting ahead of yourself. Saying that it is well established that Thunder Warriors are in many ways prototype Astartes, doesn't really add anything to this discussion, which is whether the thunder Warrior had geneseed. The evidence I have given is that the thunder Warriors had a Betcher's Gland, which suggests, but does not imply, that Thunder Warriors had geneseed. If they had similiar organs, the question becomes, where did these organs come from?
1) It could be an early model of the reducter. An early model could easily be 'primitive' in comparison to modern models whilst still not being older than Astartes themselves.
2) It could be a home-made reducter. Again, clearly primitive but not dating to before the Astartes.
Again, IF the Thunder Warriors had geneseed, then they would NEED a Reductor.
Why do you think it will stop their decay?
Both Arik and Ghota were dying. Their skin is decayed and held together with sutures and wire. What is important here are the experiments that Arik is doing in his laboratory, and apparently has been for decades. We aren't told exactly what he is creating, but his failures mimic his own condition, which is that is body is decaying. One failure is describes as riddled with necrotic tissue, the tissue not able to distribute the super-oxygenated bloold efficiently. I would suggest he is trying to create a Haemastamen organ.
At the end of the story, after he and Ghota are able to obtain Astares geneseed, he is finally successful and the two organs glisten bright red, with no signs of necrosis. I don't have the book here, but he tells Ghota that it means LIFE, "my son".
Arik is very smart and once the organs are implanted, I believe that he will be successful in halting the degeneration of his body and Ghota's.