The reason I believe Feral are only saying soon, is that the DLC is due to arrive alongside a 'compatibility patch' which 2K are saying will have: It will be available as part of the Reinforcement Pack or '$9.99' by itself.
XCOM 2's new DLC named Alien Hunters is due for general release on May 12th, but it's possible the Linux release may be delayed as Feral Interactive are simply saying 'Soon'.
