Sorry I should have actually pointed out in that post that I was only talking about Filco keycaps.
Filco use some weird 'UV coating' to give their keycaps their smooth texture. It's this coating that reacts very badly with isopropyl alcohol and the reaction causes the keycaps to turn a splotchy grey. I ruined an entire set of keycaps on my Filco because of it
I believe Ripster makes mention of it on the geekhack 'All About Keys' wiki somewhere but finding info on that wiki is like drawing blood from a stone sometimes.