How do we even know if its excess? We weren't there to watch them having sex. There is a line between having sex and excess in the form of Slaanesh. It's a natural reaction to try to distract yourself from impending doom. I understand you mentioned that this is a thin thread, but I have to say that it is very thin. I have a hard time holding any suspicion over that.
Well I know they are having a party to distract themselves, in that situation excess is kind of the point because it is distracting. I think this can be both the normal reaction and used as a focus for dark magic by a bad actor if they carved the right symbols around the place. I am not saying their actions will spontaneously spawn daemons or anything, just that they might be cooped if there is a traitor among them