What does "pitch plus power equals performance" really mean?
A.  It simply means that the airplane will perform according to the combination of pitch and power control inputs that you make.
B.  Yes, why wouldn’t it be?  Aircraft are subject to basic laws of physics.  These laws are consistent.  Every time you pitch nose down and add power you will descend and accelerate.  Every time.
C.  N/A because it is always true, but it is just a saying.  If you find the concept useful to enhance your understanding or explain something, then use it.  If not, then don’t.  Aviation is full of such sayings to help pilots remember things, like “aviate, navigate, communicate”, “ARROW” etc.  Pick the ones you find helpful and don’t overthink or try to discredit the ones that aren’t useful to you.