Considering that diesel is like a lubricant for the engine during its operation, the engine probably need to spray a minimal amount of diesel fuel per cycle to keep its cylinders well lubricated. While the amount of fuel needed per cycle may be smaller than the amount to idle the engine, considering the higher RPMs involved while coasting down a steep enough hill, the minimal amount of fuel at higher RPM may just be greater than the amount of fuel needed at idle for the same distance.
So, the above statement about it depending on how steep the hill is correct, however, it isn't as simple as finding as steep of a hill as possible to coast down from. IMO, more than likely, the amount of fuel used coasting down in gear in most circumstances will be higher than it is in neutral.