I disagree with this. It's true that, for many questions, D&D maps to a specific system.
But it may not always do. There could be cross-edition questions (about, for example, how to make halflings interesting in D&D, or how D&D has developed across editions).
And, for the moment, I think it's better to overtag than undertag. If, later, we find that dnd is a useless tag, we can synonym it then. For the moment, let's keep our options open.
That said, I think a "fantasy" tag, meaning any fantasy rpg, is a good idea. We could use it alongside the dnd tag.