Very good question. Spectral peaks also appear in random data when not enough samples are used. This is the case here, so the statement in the manual is not sound. The spectrum function here uses only about 500 samples, and thus is not really suited for proving that a spectral inefficiency exists. It is only for finding the peaks of the current snapshot of a spectrum, under the assumption that those peaks already do exist.

This is due to the nonstationarity of price data. We should look deeper into this and provide a function that samples a larger amount of data for the spectrum, so that it's still quasistationary, but can be distirguished from random data.