It depends in part what you consider to be part of "the rules". Is a flumph part of the rules? It has game statistics, which are complete nonsense without the rules, and a description that goes with them, without which again, the stat block doesn't make a whole lot of sense. The rules cannot be used without the descriptive context. So, I don't feel there's a clear dividing line between "fluff" and "crunch". The barrier is rather diffuse.
So, I end up thinking that if it is in the rulebooks, it is "in the rules". If what is in the set of books, in total, presents something that fits the bill, then it is D&D. Now, "fits the bill" is entirely subjective. I, personally, don't think what is in the Dread game fits the bill of "D&D". The experience is too different. I am perfectly willing to allow that, to some, the experience of all games really is all the same. To me, that sounds like saying all foods taste like hamburgers, so we might as well call all food hamburgers. I find it difficult to wrap my head around that.