I and along with everyone else posting in this thread have no idea of the full facts in this oversight.
My guess is and it is only a guess and l hope the following is what did happen.
They carried out the AD by embodying the Service Bulletin as detailed in the SB.
This included the functions or operational tests after the changes,.
However the FAA can not see individual sign offs of these tests, so UA is unable to prove that they were carried out, hence out of compliance with the AD.
I hope that this is the case, as no respectable engineer would not carry out Function or Ops tests after a mod of this magnitude.