I think it depends on the consequences of getting caught. What if the PCs fail to kill their boss?Perhaps what you are missing is the actual quote that brought up the DC 20 Cha check:
And the actual rules being quoted (DMG pg 245):
DC Hostile Creature's Reaction 0 The creature opposes the adventurers' actions and might take risks to do so. 10 The creature offers no help but does no harm. 20 The creature does as asked as long as no risks or sacrifices are involved.
Your desired result of "We don't' kill you, you don't kill us" sounds like a modified roll of 10 or better to me, if a roll is being called for by the DM.
I'm not arguing your overall point, I am more re-emphasizing my position that this isn't a really good example for a skill check. I don't think there is a lot of uncertainty, given the GM knows what the contextual details are behind such a decision.