I don't think he says that Snape was popular, exactly -- just that he was "famous" for his interest in the Dark Arts, and that he was "part of a gang of Slytherins who nearly all turned out to be Death Eaters."

But yeah, I have a feeling JKR has forgotten all about that gang -- which is a shame, since Narcissa Malfoy's 1975 was one of the first really good fanfics I read, and I've had a soft spot for the whole crew ever since.