Note the words should and recommend. It's not official at all so if you go into tournament and opponent decides to aircraft snipe there's NOTHING YOU CAN DO short of packing your miniatures and giving your opponent automatic maximum victory.

If you make TW's effectively invinsible until rest of the swarm is destroyed or cripled don't be surprised if you see aircrafts being used as workaround. TW's already take more firepower point for point to take out than hive tyrants. Make them even tougher and people will start looking at new ways to kill 'em.