Actual artificial difficulty is basically when you jack up the health and damage an enemy does without actually making it any more difficult to fight, or possible to defeat using tactics/intelligent play. A good example of this is what they did to the HW overworld where every mob really isn't that different from the mobs you fight from 1-50, they just have massively inflated health pools.

The term "artificial difficulty" has been thrown around since Dark Souls became popular, and basically people just use it to mean "I don't like this mechanic" now.