It's the other way around. There are elements of The Dark Tower in Rose Madder. That happens a lot with the Dark Tower related books. There are three types of Dark Tower related books. One type are the books that effect the Dark Tower story. The Stand and 'Salem's Lot are the most obvious examples, although The Eyes Of The Dragon, Everything's Eventual, and Hearts In Atlantis qualify as well. The second type is what Rose Madder is. Nothing in the second type effects the seven books in the series, but what happens in the seven books in the series effects the second type of story. Desperation, Bag Of Bones, and The Regulators are more of the second type. The third type is a mixture of two, in that a lot of Tower mythology occurs, but it's not actually central to the seven books. It, The Talisman, and Black House are the biggest examples of this.Havent read Rose Madder yet, but weren't there elements from Rose Madder in the Dark Tower series (in the list of previous king novels in his books, DT-related books, including RM, are in bold)? If so, he hasnt completely disowned it.
Basically, the fact that The Dark Tower can be found in Rose Madder, but not the other way around, is why King feels comfortable disowning it.