My issue with this metric is that it is completely irrelevant, I see users posting replies like "lol, right", "me too", "hi", "you're welcome", and so on. Which are fully useless, add nothing to the topic, yet increases the activity.
No they don't.
The activity is the minimum of two values, one of them being the post count and the other is time-based. If you post a lot, the minimum will usually be the time-based one.