If you want to speak exact, then it's assumed by the players.
It might be just the way it is, it might not be but it doesn't make "sense" in my personal opinion.
He has a shield, we would assume he would benefit from shield rating and absorption but he doesn't (besides his own tank stance buff). He benefits from defense rating (aside from endurance and armor..) .. but that can't be the only mitigation stat because it "seems" absurd.
I mean the developers could've just designed him in a way he would be better off as a DPS and let Khem Val be the glorious tank.. who knows? But based on how assassin tanks work, I would also assume Xalek would be the same.. but also.. who knows? He is.. afterall a companion.
Blahdeedah.