The greed CD pens actually work, by the way.
Derisive comments serve only to burnish reputation among those who find it easier to dismiss out of hand than do the work of finding out for themselves. Credit and props to those who do put a little more effort into doing audio than expounding on that which they do not know.
I've been demagnetizing for years. Since the 90's, when you could buy the Radio Shack Bulk Tape Eraser for $20. Then much more expensive (but no better) options came along. Still use it on a regular basis.
Demagnetizers work by creating a strong magnetic field that being AC oscillates flipping directions. Held close and then slowly moved away allows the fields to slowly randomize and become demagnetized.
The XLO Test CD has demagnetizing tracks I use several times a week. Even though most of what is in the signal path is non-magnetic still there are impurities and regions that are.
The way magnetism works, a strong magnetic field can orient metals along field lines. If the magnetic field is strong enough to align and then quickly removed it magnetizes the metal by leaving them in alignment with the magnetic field. These small islands of magnetism distort the music signal. This is pretty much what all music is, powerful dynamic transients that come and go. Over time these islands of magnetism build up.
The idea is playing a tone that slowly fades out is analogous to using a hand held demagnetizer. Another track sweeps from low to high frequency, accomplishing the same trick but across a wider frequency range. The XLO disk has both a steady tone that fades and a sweep tone.