Well, if you want to be 100% accurate, I think the section talking about how the high notes aren't important could be clarified. The really high "notes" have already been lost when you recoded the wav file digitally. The lossy step of mp3 encoding is not a result of the transform, but what you do with that information and is more complex than just discarding high frequency components.
Also, the the word "note" is confusing in the context of music, since really low notes usually contain a lot of high frequency information.
Correct. (For the author's benefit) We usually call them harmonics, in the context of pitched sounds, but more accurately, the sinusoidal components of any sound are called its partials. Partials differ from harmonics in that harmonics are restricted to be sinusoids with frequencies that are integer multiples of a fundamental frequency. Real world musical notes don't often exactly fit this paradigm [1].
In any case, if discarding high frequency information is all you needed to do to compress, you could simply low-pass filter the time-___domain signal. A better description of what goes into MP3 compression is that it omits frequency components in sound that we can't hear because they are shadowed by nearby (in time and/or frequency) components that are louder.
Also, the the word "note" is confusing in the context of music, since really low notes usually contain a lot of high frequency information.