Roger that. My understanding is that Chaos Warriors, regardless of mark, come from StD, and are battleline StD. Having the Tzeentch keyword just keeps them in line with getting Destiny Dice. I could be wrong though, and defer to someone who has better understanding of this. Also, if someone could cite the page or faq that clarifies this I would appreciate it.
Edit: I know this is very rules lawyer, but here is my logic. Page 311 of the core rulebook states "Allied units are not included when working out the number of Battleline units in the army." On page 66 of the GHB, Chaos Warriors aren't listed as Battleline, under the Disciples of Tzeentch. Then on page 68 they are listed as Battleline under StD. Page 69 shows the allies charts, and Disciples of Tzeentch has StD listed as an ally, excluding mark of Nurgle. So I could take StD as mark of Khorne, Slaanesh, or Tzeentch and they would be allies. Based on this, I think they are allies, and can't count towards Battleline requirements.