A creed is NOT Scripture. It is not a complete statement of everything that we believe is Christians. Frankly, it doesn't need to be because we HAVE Scripture. So, even though the creed doesn't have a statement on the atonement, it matters not because I do not take the creed as authoritative. Rather, it is a statement of faith. Incomplete, but valuable for a general understanding.
Why make additional (imperfect) statements when all you need is the Scripture? In fact, why do protestants write so many books about what the Bible says if all you need is the Bible?