In this situation you forced them to forfeit their entire moment of surprise in order to turn a 3 on 3 into a 2 on 3. Even this isn't guaranteed, as a defensive cooldown on the chosen target can make a kill uncertain. They lost the possibility to do anything along the line of mezz->flashbang to get a tap in. You wont win a 2 on 3 in an "open field" situation, but while defending a node it is usually sufficient to get support to the node you defend, esp. in alderaan where the person you lost will be able to fly back to the node immediately. From this point on the supprise attack has become the default node zerg which (in my experience) favors the defender. Exceptions might apply to novare coast, where getting additional defenders to the far node can be problematic.
In contrast, if two people left to chase one person, the node defender might find himself mezzed, the first stealther starts tapping, the initial mezz forces cc-break on the defender and cc by the second stealther stops the defender from preventing the tap. The two people that left have 8 seconds to return (and get into LOS), with two people (the bait and the second stealther) free to snare/root/stun them, depending on the setup. Of course, this requires a lot of coordination on the attackers part and isn't foolproof by any means, but I've seen it succeed more often than a head-on 3 on 3 attack...
In theory 8 secons might sound like enough time to bridge something along the line of 50 meters, but just as a significant number of classes has suitable gapclosers, a just as signficant number has instant snares/roots. Even if you break cc immediatly, you're likely to lose a second or two. In addition, in order for you to rely on your gaplcloser, you can't use it in the initial attack on the bait, e.g. as a marauder/juggernaut you can't use charge to get to the lone person. Relying on the gaplcoser also means you need line of sight to the person trying to get the tap in, which isn't guaranteed either (a stealther trying to get a tap on an alderaan node will usually position himself behind the node)
The point you're consistently missing - the point of this kind of trap is not to attack a single person with multiple stealthers but to crowd control him so that getting a tap in is possible. It doesn't matter how hard it is to kill the defender, the attackers goal in this situation shouldn't be to kill but to disable him.