In that case, I definitely think if the person doesn't reply AT ALL to a log that they were tagged in, it shouldn't count towards activity. In cases where it's one reply that isn't very long, that gets a little dicey too...

I can see why mods wouldn't want to look through every log to see who commented, but wouldn't it be easy to see from the number of comments if that needed to be looked into? (Two or three comments might be worth opening to see how long those comments were?) Maybe that's an easier way for it to be handled.

I know that it's a hard thing to gauge, but hopefully there's a way to do it that won't be too much of a headache for anyone.