Whether it's a PM or the regular posting activity, the forum backend will still detect it all as part of the users activity in the system even though the PM activity isn't counted on the users activity.
It's being considered by the server that such user wants to execute a communication activity so, it can't be compromised of it timing system on this course.
Moreover there's no essence of emergency in this scenario as the forum is concerned because the Mod may not be 24/7 active to reply you PM so urgently. So don't be hasty to find this as burden.