I’ve actually used copper slip for around 50 years with no trouble !!
Me too (well, maybe 10 years or so) and no problems and prior to that, ordinary high temperature, general purpose grease.
Copper grease is an anti-seizing compund rather than a lubricant and is used in a wide variety of engineering applications. Yes, it does contain small particle of grease and maybe the warning above comes from the idea that dissimilar metals when together can create a problem.
A bit like the 'which oil is best for my car' discussion, opinions vary. In choosing whether to grease the freehub splines or not, I have all my cyling life greased them in the belief that if there's grease in there then that reducrs the chances of 'other stuff' getting in there. (And I also lightly grease the square tapers on the bottom bracket which is another often debated point, and don't forget the seat post unless carbon).
Now my approach may or may not be right - all I can say is, I've never had a cassette (or crank or seat post) seize up on me, so there'll be no change of habit in this guy's bike shed