Below you can see my friend pull a mob and gain initial aggro. They do not attack or generate any threat yet when I hit with a Holy Shock (roughly 600 threat) the threat does not register, but when I cast a Seal of Righteous (roughly 60 threat) the mob then starts attacking me. This proves that Holy Shock threat is not being applied unless I am on the threat table and that my friend only had initial threat since a single Seal of Righteous pulled threat onto me.
I have also tested this by blessing my friend after they do 1 auto attack so they keep aggro but I get onto the threat table, and when I Holy Shock the threat gets applied.
Is there a reason why Holy Shock does no threat unless I am on the threat table?