I think that version of the stun icon means that unit can't be stunned (some big/special enemy types are passively immune, others can get a buff from casters that prevents stun)
I had a hammer by endgame that did 5 Stun on its Force Strike so the guy I gave it to could just run around turning every regular plague marine that spawned into a chain of AP pinatas
Me: /throws one grenade
Game: "Objective complete"