Dictionary definitions are not exhaustive. Otherwise, we couldn’t say that women who had double mastectomies are flat-chested (though to be fair, calling them that wouldn’t be a good idea).
The breast tags are meant to be a continuum. Completely flat is flat chest, the smallest discernible breasts are small breasts, and so on. The character has discernible breasts, and since small breasts imply breasts, which should not be used with flat chest on the same character, flat chest does not apply to her in this post.
