From a Wing Chun perspective, I see attacking the core as focusing a strike into the motherline of the opponent, or moving through the motherline of the opponent with your footwork. I see now that you have referred to manipulating an arm as also attacking the core. While I agree with this in essence, I don't think it is how most Wing Chun people would view it. After all, any Jiu Jitsu or Chin Na technique ever devised also "attacks the core" because they are going to off-balance the opponent in some way and make him move his motherline. Every Judo throw or Silat sweep is also "attacking the core" for the same reason. So really, seen from such a broad perspective, there are very few techniques that wouldn't "attack the core."
So focusing on the weapon arm to control it while off-balancing the opponent in some way definitely attacks the core. But it is indirect.
Unless I have misunderstood what you mean by "attacking the core"?