If they are not interchangeable why do so many restaurants say 치킨 when it's not fried?
Maybe there could be one whose owner wants to sound exotic, but there can't be many. Calling every kind of chicken 치킨 would definitely confuse most people, and is also incorrect unless you're just transliterating chicken into Korean.
닭고기 is chicken in general, and 치킨 is the fried chicken Koreans are so obsessed with.
it might be that 딝고기 is the native word for poultry and 치킨 is just the konglish anglicism they picked up.