Didn't get that was what you were talking about. (That required reading details of the DS.)
Well, I don't know how we can answer your question really, just ask Cirrus?
That said, other parts of the line do measure apparent, active, reactive power, the power factor... that looks adequate for many uses in power supply monitors to me, so I'm not too surprised that the additional harmonic measurements are not available on all models.
What's your application and what would you use a specific harmonic active power measurement for?