IIRC from some article or interview way back then, Martin had provided D&D an outline of all the major plot points he intended through the end of the series. So while they might not have had the specifics, the major points would have been there.

If that was true, then it would make sense that they used those major points for the basis of the rest of the show. After the abysmal reception of those points by the fans, I would imagine Martin would have stopped to think about his plans, possibly losing interest entirely.

That assumes that he did in fact provide those major points to D&D in the first place to have adapted however.