Actually the http://forums.impulsedriven.com/323756 article that you reference is the same article as https://forums.galciv2.com/323756.
That is because the Impulse driven forum "Home > My Games > GalCiv II > Twilight of the Arnor" is actually http://forums.impulsedriven.com/forum/483 which is the same identical forum as the GalCiv2 forum "Home > GalCiv II > Twilight of the Arnor" or https://forums.galciv2.com/forum/483.
Also the GalCiv II forum in each path is identical, in one case referenced as http://forums.impulsedriven.com/forum/162 and in the other case referenced as https://forums.galciv2.com/forum/162.
Note that this is how all forums that exist in multiple sites work, however this only works for forums that exist on both sites. For example the "Home > GalCiv II > Metaverse" forum or https://forums.galciv2.com/forum/345 does *not* have a corresponding counterpart on the Impulse driven. That is http://forums.impulsedriven.com/forum/345 does not exist in reality. If you click on the forum link for a forum that doesn't exist you might end up in some kind of inter-forum limbo that isn't related to any obvious place in the site in question. In other cases you simply get a 404 page unavailable error.
However while you can't see forums from one site that aren't implemented in another it does seem that you can always see any article at any site even if that particular site doesn't happen to implement the forum that contains the article. Caveat Emptor, I by no means have exhaustively checked this out but so far it seems to be true.
For example going back to the Metaverse forum which is a subforum of the GalCiv II forum (i.e. /forum/345) and is present on forums.galciv2.com but not on forums.impulsedriven.com. However https://forums.galciv2.com/154389 which is Article 154389 which exists in the GalCiv II > Metaverse (i.e. /forum/345) forum also is accessible from forums.impulsedriven.com as http://forums.impulsedriven.com/154389 even though you could never actually find it listed anywhere on Impulsedriven.
Not sure what real benefit any of this provides other than a glimpse of how the underlying structure of these inter-related websites work.