If the author of HD 2K were to fix it, what would he have to do?
Depends. If the author actually has all of the elements (all four channels), they'd only need to redo the process of creating normal maps from them. (And not mess it up. ;p ) The few specimens I looked at, however, only had one usable channel and the hint of a second. The third vector channel and the reflective channel were both copies of the vanilla originals. So one of two things happened: Either the author legitimately screwed up, and then ultimately either never noticed or never cared enough to fix it, or they never actually created usable normal map elements in the first place, and their hybridization effort was in fact deliberate (and effective) subterfuge. In the latter case, "fixing" the problem would require nothing less than starting from scratch and handcrafting every single normal map, as should have been done in the first place.