They were likely talking about the bimetal jacketing on the milsurp and new foreign ammunition. FMJ is not necessarily hard on the barrel. It just depends on what the jacket is made of.
Some American manufacturers use a copper jacket. Wolf makes a copper jacket as well, although it's a "specialty" ammunition. Their standard fare, and even military surplus, uses a bimetal jacket that contains other metals like nickel, etc.
However, the SKS was designed to use bimetal jackets. There is nothing wrong with it. The hardened steel in your bore is made for it. Using bimetal jackets is not going to be an issue unless you're one of those guys who shoots a thousand rounds every weekend or something. Otherwise, your SKS bore will likely outlive you and your children as long as you take care of it.