I would argue against this. It depends on the goal of the forum.
If a mental health community has the objective of saving lives then I would suggest that people logging in and reaching out to others privately are very much engaged in the forum.
I agree that PMs shouldn’t be part of post/topic count stats but I do think that people that write or respond to PMs should be included in an engagement stat.
I think in most cases they should though. There are communities where people engage via email alone. Not many, sure, but those numbers are fundamental in those cases.
I hear you on this but I think this is an edge case.