Why do people pretend like a 5.56 and 223 aren't the same round?

I was talking to a friend and said I'm going to get a 5.56 upper for my AR but he said I'm lying because you can't get a 5.56 upper cause the round doesn't exist. I said what? He said a .223 is 5.56 but a 5.56 is .223/5.56 (a 223 situation but boosted). The round is 5.56 but within that you have .223.