I would assume that many things can be demagnetised, the reasoning for demagnetising would be the more difficult reason to comprehend.
I am happy to leave the Math to those that fully understand it through creating the equations and not just merely quoting it.
The Clever Types can produce devices that cut to the quick and do what they say on the Tin.
A Degauss Device is quite affordable in some cases cheaper than a Device to control Anti-Static on a Vinyl Album and is especially cheaper than some of the SUT's and Head Amp's that can be purchased.
I have heard the positive outcome of a Degauss being administered and as stated in some cases the improvement heard has been quite notable and definitely worthwhile.
It really does not make sense to have multiple Thousands of £'s invested and not give the full electronic support to the critical devices used in the Phono Signal Path. It is not uncommon to see a Phon' Pre- Amp' set up costing close to £/$ 5K, to be used with Cart's costing upward of £/$ 2.5K.
A mechanical device to support the Cart' and serve as a conduit for a Signal Path can quite easily be seen as a cheap option at £/$ 4K.
Spending £/$ 200ish on a Degauss Device does not seem to be the purchase that is a ninth-degree approach.