They generally are somewhat predictive, granted this is just a single data point, so by itself it means nothing, but a 20 point swing in a solid red district isn't comforting for republicans. In the end it doesn't really matter who wins one way or another, this is mostly about a bit of a forecast of how 2018 could play out. Assuming this is indicative of trends for 2018, having it happen this early is actually good for republicans, by the time dems started losing special elections in 2010 it was too late to course correct or try to salvage anything.