That build uses Lokarr Set to do Gladiator Crucible 150-170 without blessings or banners, kill Ravager and do SR 75-76 on Ultimate.
You don’t need armor on pet builds 
Pets aggro enemies by default. How much the value is, I am not sure. But different pets generate different amount of aggro/threat.
For example, you will find Briarthorn even without Ground Slam, holding aggro better than Familiar or Reap Spirit.
Now, Hellhound, Briarthorn and Blightfiend have special skills that generate aggro. The more point you invest into these skills, the better they become at holding aggro.
Taunt is a bit different as it more or less forces enemies to go after your pets instead of you whenever it procs, instead of simply generating threat.
Think of it as the highest level of Threat or Aggro your pets can generate. Outside of Ishtar’s proc, the only other one ingame is Briarthorn’s Ground Slam at 17 points. It changes from Generate Additional Threat to Taunt at 17/12.
However, bosses and such tend to have resistance against taunting. So that is also something to keep in mind. You can look up taunt resistance for enemies in GT’s monster database.
For example, Gravathul has 45% taunt resistance: https://www.grimtools.com/monsterdb/1154/skills
While Ravager has 75%: https://www.grimtools.com/monsterdb/364/skills
I am not sure on exactly how all these things interact on a mathematical basis, but in practice it just means that a Briarthorn with 1 point in Ground Slam (GS) will hold better aggro compared to a Briarthorn with 0 points in GS and a Briarthorn with 10 points in GS will hold aggro even better and at 17 points it becomes Taunt and hence much more effective.
And compared to Ravager, Grava will be more inclined to aggro on your pets most of the time due to lower taunt resistance.