True, modern nickel plating does not use an substrate of copper, but I don't assume anything about nickel plating unless I know about a specific gun. And it doesn't take any obvious nicks or scrapes to allow the copper solvent to get under the nickel; it can happen at the muzzle, on a cylinder face, around a sideplate, in an auto where the slide rubs on the frame or where the hammer rubs off the nickel, anywhere.
The chemical is ammonia, long used as a bore cleaner to remove cupro-nickel fouling and copper washing from the barrel. (Hoppes No. 9 is one of the oldest, but there are others.) If it can dissolve copper in a barrel, why would anyone think it won't dissolve the copper under the nickel?
I once got into a discussion that ended with calling Hoppes; their customer service woman did a song and dance, but finally admitted that the product should not be used on nickel plated guns.
Jim