Typically, flat-chested girls do have a nipple that looks swollen or elevated, but from "flat-chested" teenagers who are very worried about their breast growth.
It depends on your definition of “flat-chested,” but generally there are very few young girls who have truly not grown any breasts. Do grown/adult women who are completely flat chested in anime actually exist in real life? Would you consider marrying a lady who is considered flat.
I'm slim with a nice bum but I have a really flat chest, like I'm talking 32A and not- needing-a-bra flat. Think Keira knightly. Even though I'm quite comfortable.