Because what is self-contained?
Altimeters. They do not depend on bits outside the aircraft, same as INS.
You're not familiar with true altitude?
Nope. It might be a standard term; heard and perhaps used it before, but I don't recall ever seeing a definition for it, and since there is no such thing as a "true" altitude unless conventionally defined...
True altitude is actual altitude. It is calibrated altitude, corrected for temperature
Note that the above is still
not "actual" (geometric) altitude unless you have a temperature profile for the whole column of air beneath the aircraft [and gravity readings, I might add]. I am not disputing that it might be called true altitude though. I might have to dig out my ATPL notes
Suppose that should read QNH?
No, QNE is correct--it refers to standard pressure, as in setting the altimeter to 1013.25 to fly flight levels.