This is precisely the point: even though BaN has no other AV, this does not make it an exception to the rule through having nothing that shows its root, for it certainly does have something that shows its root. Except that in this case, what it has is only the fourfold form of the simple Havayah. What AV, SaG and MaH have over and above BaN is the fact that besides the AV contained in their fourfold form, they also each have their own particular AV in their filled or expanded form.
Thus the intent is properly fulfilled, showing that no branch can escape being included under Arich Anpin, which is its root. For even BaN, which does not have its own particular AV, nevertheless does at least have the AV that exists in it by virtue of its being a Havayah (whose simple, unfilled, fourfold form = 72 = AV). The reason for this difference will be explained below.