I get what you're saying, but the definition of the term 'prominence' doesn't help you, because by definition, Mount Everest's 'prominence' equals its actual height above MSL. This special case is necessary to give the recursive definition of 'prominence' a base case.

However, it's true that Mount Everest's "relief above local terrain" is not equal to its actual altitude above MSL, but this is just informal empirical footballing and no rigorous technical definitions exist.