What's your favorite food? If someone gave you a little of your "second" favorite food multiple times throughout the day, but your favorite was sitting there in front of you the whole time...
They nip at corals because they like corals.. doesn't mean they won't eat other things, but they like corals. There are various degrees of reef safe, and I've seen posts of angels that don't nip, or whose owners think the nipping improves growth... but the like corals.. so with an angel there's not training them to not like coral, you might get lucky, you might not... risk/reward