A few things.
Heya. What I meant to communicate is that the silence only procs from Direct Damage spells, including the arcane-dmg weapon-imbue-type that the Suramar City mobs use. Pretty certain: dots and AOE never proc it.
Testing:
Shield’s Rest WQ: Good for test: two types of elite caster vrykul, one priest and another similar. Both have DD spells and a nature DOT...'Choking'-mist?. They're spread out, so easy to 1v1 for careful monitoring. I'm in tank spec with 3-4 interrupts besides the trinket, so I can block most DD. Killed a dozen in lengthy exchanges, and they'd get silenced from DDing me but never from DOT-ing me.
Argus: Felsingers: DD+Rain of Fire. Killed a dozen, same story. Puscilla’s imps' puddle damage: more difficult to test, but again never saw them silenced.
Good news: it does silence Elites in Temple of 1000 Lights, both the familiars and the mages. The familiars may have a dmg aura, but if so, I didn’t see any silences from that. Also, some Elites on Isle of Wardens were silenced, when they used Howl/Roar-of-something.