That saying is bullshit. The purpose of a system is, by definition, what it is intended to do, not what it does. You can judge efficacy by the results, but not the purpose.
No, the entire purpose of the POSIWID principle is to refute what you just said. Do read the wikipedia page linked above. You called it out yourself when you said "by definition" - there is definition, and then there is reality, and they need not align.