The Culture explores 40K II: Now With 100% more Fanfiction

[Spamotron]

One X factor is that the barrier between Warpspace and Realspace is likely especially thin here. Daemon manifestation and summoning could be much easier than usual and as noted the RT's forces lack the necessary punch to bring the Chaos ships down quickly potentially giving them enough time to set up a proper summoning. Once it becomes clear to the Chaos forces that A: They can't win this fight conventionally and B: That the RT ships are a truly exceptional prize. They might be willing to risk such a thing. Of course its a roll of the dice summoning a Daemon that is both powerful enough to take out the RT ships and can also be reasoned and bargained with enough to take the RT ships intact and not just eat everyone.