I'm not sure the above statement actually works in anything more than concept.
We've all seen the many multiple posts over the years complaining about certain quests - You know the ones...Those marked Challenging!
Players don't like seeing a Level based difficulty setting AND a C/N/H/E based difficulty setting that is also in some way tied to actual levels suddenly thrown out of the window in certain quests!
Proof is in the Poison may be a Lvl 4 base quest, 5 on Hard, 6 on Elite and people can understand that.
What they don't understand is why that quest is so much more difficult than 90% of the other Lvl 4 base quests, 5 on Hard, 6 on Elite!
A Killer DM difficulty applied to Proof would possibly count it as a Lvl 7,8 or even higher Quest for that difficulty but people would still have to run it at Lvl 6 or lower for BB.
How many complaints would there be do you think?