Legendary node No Witnesses says Ultimate Skill does 30% increased damage but on the Death Trap’s tooltip there is no change in damage reported.
Your Ultimate Skills deal 30%[x] increased damage and grant you 10%[x] increased damage for 20 seconds when cast.
Check if you get a buff for 20 seconds.
its hard to really test without target dummies but as it says;
“Deal more damage” not that it increases skill damage.
I think its just not calculated into tooltip.