I don't understand why a Moderator for a forum would want to suck members and activity out of it to a another site?
Forums are a zero sum game. any activity that one gains is lost to the other.
posts beget posts and posts lost to that other forum will never come back.
On the contrary that other forums success will suck activity out of this one.
A generalist site such as this one is particularly vulnerable to have its sub forums destroyed by people leaving to specialty forums.
If I was the owner of this site I would be very concerned.