Those theologians who advanced the theory of limbo did not advance it as a part of hell, but rather as a part of heaven, short of the beatific vision. They wanted to keep hell unified (as punishment for sin) but were willing to divide heaven. Ratzinger was saying that both should be kept unified and limbo makes no sense.
I think you may have incorrect information. I have honestly never heard any theologian say that, and I've read up on this somewhat intently, though certainly I'm no expert and am eager to be corrected.
Dante quite clearly sets Limbo in his "Inferno", and every theological treatise I've read on it, unquestionably defines Limbo as part of Hell. It has to be: the Limbo of the Fathers was postulated to exist before Heaven was even opened.