Now it gets a little interesting.
You see, when most people use 'griefing' to refer to computer games, they don't necessarily mean 'exploiting' or 'cheating.' Those words are perfectly viable. What they mean is that they are being frustrated by someone.
Even so, provoking flags through clever positioning tricks IS exploiting the game mechanic... So that would qualify as 'griefing' according to that link of yours.
And now there's a little grey area: Killing questgivers in a lowbie area. While killing off the NPCs itself, in a vacuum, is not griefing per sé, the kicker is as follows: If you do so with the intent of preventing other people's progress/gaming experience, if you do so with the intent of frustrating them, then you are exploiting the game system in order to frustrate people. The game system allows you to kill NPCs. There's nothing wrong with that per sé. But the secondary effect is that other players will not be able to take or hand in quests... The secondary effect, that becomes the main objective. So yes, you are exploiting the game in order to frustrate unwilling participants, so yes, you ARE griefing even by gamer lexocon definition.