This is wrong in several ways.
First, Sentinels are not Ranged Tanks, they are Ranged Scrappers. They do less single target damage than Scrappers, but tend to do slightly more AoE.
Second, telling new players in a new player guide "nobody plays this" is not good guidance. If you don't play them, that's fine, ask someone who does what is fun about them.
Third, Vulnerability is NOT passive. Sentinels passive ability is called Opportunity, and gives them slightly increased Perception, an increased Perception Base and Cap, a little Perception and Tohit Resistance, and fills their power meter passively over the course of a minute.
They can spend half of the meter to ACTIVELY cast Vulnerability, which is an autohit, no endurance cost, almost no animation time ability that debuffs Stealth, Defense, and Resistance to almost everything, and that is not reduced in strength by enemy level.