Look they aren't taking about in the heat of battle. Yes you can stealth with any cover/concealment. IE hiding in a dark ally waiting to ambush the poor sap leaving the bar.
No you can't attack from that darkened ally, round after round using a stealth check after you shoot him. You shot him now you can snipe or be out of stealth, he see's you, you are observed even with the 20% concealment you can't stealth because you are being observed.
You could however round a corner out of sight and restealth in combat because you can't be observed around a corner. If you want to restealth in combat, id like to keep on point with that fact of the thread. You'd have to either bluff (distract) them or use the snipe skill so as that they never saw you in the first place. I mean Aware of location, they saw you shot or stab them, they are aware of your location.