it clearly says, and I quote:
" An Officio Assassinorum operative may only be chosen if an Inquisitor Lord or Inquisitor is also part of the force. Note that no more than one Officio Assassinorum Operative can be used in any force for any reason. "
It doesn't say that only DH armies have to take an Inquisitor. It clearly says that any army must have on in order to take Officio Assassinorum operatives.