TL;DR - Mercury has long been considered a "conventional superconductor" due to having Cooper pair interactions, etc. but the basic DFT models that usually predict critical-temperature for those kinds of superconductors have trouble making good predictions for Mercury. Now, using more complex modern DFT models, they're able to get a critical-temperature prediction from the theory that matches reality to within 2.5%, as well as pinpoint which aspects of Mercury throw-off the basic stripped-down models.