The very fact that this correction had to made offers some insights into the National Security Agency.
I believe that releasing DES to the public was the biggest cryptography mistake that NSA ever made. Consider the state of research in cryptology before DES. It was simplistic. It was haphazard. There was little interest. If any results of value were ever discovered, the NSA could squash them with a secrecy order. No one cared.
There is one problem with this analysis: IBM created DES. Not the NSA. Sure the NSA could have asked them to keep it hidden, but the NSA was also going to IBM and warning them about Russians evesdropping on IBMs networks. Everyone realized it was time for public cryptography. Especially IBM. It is not clear that a secrecy order would have worked. This is not to say that your analysis is wrong. They classified the design procedures which was their attempt at a compromise. IBM couldn't publish the details of how to make a good algorithm, but they could release the details of the standard.