While it's true, they only took the majority of vulnerability up in Tender valley. In Alexandria, 2/5, Straighbourg Deadwalk 1/5.
On other content they've cleared I've seen the mitigation being used, so I just assume they went in those dungeons with shit&giggles in mind.
In any cases, Samurai survives 5 vulnerability stacks with 3 from the last boss alone without the need for a Warrior, Paladin or any healer.