So I noticed today that the Tactics talent Shock Absorbers, which reduces the damage taken by 30% whilst stunned, does not appear to increase Damage Reduction in the character sheet, nor displays a buff icon when active (which I would expect as feedback to the user if the character sheet does not reflect it).
Not all damage reduction in the game is added to the player's damage reduction. Warding Call, the Guardian tank damage reduction CD they get in their tree, doesn't add 40% to their already impressive DR; it simply reduces whatever damage they would take by 40% (so if, after all other sources of mitigation, they're getting hit for 100 damage, it instead gets reduced to 60). The same applies to this talent (and the various other AoE damage reduction talents that many other DPS classes get).
If you want to test whether it's being applied, get into a duel with someone you know. Let them hit you with an attack that has a small damage range then let them stun you and try the attack on you again. You should see a ~30% reduction in the damage you take between the first and the second.