take some chemistry. Some "solvents" will dissolve many things but not everything.
Every solvent is going to be either an acid, or a base, or "neutral" (which is water)
bases dissolve certain metals and don't touch others. Acids dissolve certain metals and don't touch others. It is entirely dependent on what the chemical is (its concentration) and what the metal is.
There are things that will eat copper and not steel. There are things that will eat both of them. For one example, strong enough nitric acid will eat brass in seconds, and eat carbon steel in minutes or hours but won't even etch stainless steel.
Caustic (bases) like Sodium hydroxide eat aluminum like candy. H2O2 (hydrogen peroxide) in industrial strength level will cause organics to burst into flame.
Just saying that your copper fouling remover will eat your barrel steel as a blanket statement is false. IT MIGHT, but that depends on exactly WHAT you are using and what your barrel steel is.