Ironically, if State Law simply defered to Federal Law, not only would there be no registration requirement, the serial number wouldn't be required either.
Of course trying to figure out what actually happened with third-hand info will likely yield no accurate answers, my guess is someone stated something that was later misinterpeted or ignored. My guess would be an explanation along the lines of "if this wasn't Hawaii..." where for the most part, would make the non-requirements true.
On the other hand, if it is true, we need more people like that in firearms division. Seriously. All of them would be ideal. I take it back, the whole Department would be ideal. Legislators aren't upholding the Constitution, Law Enforcement should despite that.