They have the input from all the different sensors and forecasts. Why not, for a given location, keep track of which one gave the best results? Sure, they don’t have a way of keeping track of they now, but it seems like it could be added.
This is already SOP at most reputable weather data providers; they consume many different numerical forecasts and use statistical post-processing to choose an optimal blend of the available forecasts based on how different forecasts have verified against observations.
But this sort of technique only works for medium range forecasts. Short-range precipitation nowcasts are almost always a single, deterministic run of a model that extrapolates from patterns in recent radar imagery. They aren't bias corrected at all, so you can't use observations in the same way to improve them.
