This is almost certainly a typo or a miscommunication between the balance devs and whoever wrote the talent tooltips in-game.
What I suspect this talent is supposed to do is grant you 30% bonus attack damage passively, and then share that 30% bonus damage with your teammates when you activate it.
This is what happens when I tested it in-game, so I suspect this was the intended description.
The alternative is that activating Trueshot Aura grants an additional 30% attack damage, bring the total bonus damage up to 60%, and then sharing that total with your teammates.
I am fairly certain this is not the case, for two reasons.
- This would require two mistakes, both on the part of the balance devs and on the part of the tooltip author(s), which is less likely than merely a mistake by only the author(s).
- This would be absurdly OP. 30% bonus damage on someone like Zul’jin is already a lot, but 60%? That would be ungodly levels of brokenness.