It's not the first time someone at Harley tried to pull that "won't cover more than one chrome part" horse hockey, but under current laws they can't blow this off so easily under the factory warranty. Unless they can prove the pitting is from something that happened to the bike after you took possession, like contamination by a caustic chemical that attacks cheap chrome, they have to cover the defect. If the only chrome parts on the bike that are pitting are the rocker covers, it's pretty obvious the problem is related to the manufacture of those covers and not some nebulous atmospheric pollution or other hogwash.
I'm pretty sure that one part only thing was originally created for non-warranty policy adjustments, where they agree to cover damaged, not defective, parts. Someone at H-D has been telling dealers and customers that also applies to warranty items, and it does not. Take them to small claims court if they don't do the right thing.
Jerry