Pro Tip - Everything is always Tzeentch's fault. Even if there's no way it could be.
More seriously, this makes sense. It's clearly a successful and powerful fleet, dedicated to Khorn who he does love teasing, regardless of whatever else. That fleet being lost in order to test the Culture would not be a loss for Tzeentch. Alternatively, there's a good chance that the fleet will take down one or two of the ships involved, and that means a non-zero chance of them taking some Culturetech.
With the Khorn tendancy to want to board ships, this is more likely than it otherwise would be.
I'd figure once the RT's tactics of hit and run become obvious, the trick will be to force them close. They could do this by deliberately leaving a gap or two in their coverage that herd the RT or the less disciplined mercenaries closer, allowing a teleport boarding attack to hit.
Inciting battle-lust in their attackers could be
a good tactic
idea too. All they need to do is get the idea that FTL ramming speed is a great idea into one of the mercenaries heads and things could get complex.