An inexperienced or un-optimized party could wipe on those 4 orcs. Or they could blow all of their resources and barely eke out a victory. A highly experienced or highly optimized party, with a solid grasp of tactics and clever use of resources and abilities, could steamroll those orcs with relatively little thought.
The fact that the orcs "should" consume a fifth of the party's resources don't mean that they will, and yes that is very dependent on the players' skill and optimization.
I agree, hence the "not" in the quoted bit.
What I am asking is if "an encounter that consumed 20% of the party's resources," is an appropriate challenge. Not "will a CR 8 stone giant really use up 20% of the an eighth level parties resources".