What those sentiments typically miss out on (even though we try to say it all the time) is that plenty of people cooperate "across forum lines."
For instance, to help make up for the fact that some of the "PPG core modders" have wandered off in recent years, people like Jastey and DavidW have helped update and maintain some mods recently. Over time, most of the prolific/accomplished/whatever you want to call them modders have contributed in some way to mods developed/hosted outside their "home" forum. (Sim is credited on a zillion mods. I've lent some time and voicework to non-PPG productions. Kulyok has produced popular mods on both G3 and PPG. Etc.) Although there are arguments, disputes, and disagreements from time to time, the fact is that most modders seem to feel pretty comfortable lending a hand on most forums. The divisions are mostly cosmetic, or represent benign differences of opinion, but are still valuable for the reasons I mentioned.
Like I said, I'm a little bit behind.
As far as keeping existing entries up to date, the modlist actually allows people to establish a username and password when they input their entries so that they can maintain their own listing, although in practice most people do not avail themselves of this option when they submit a link. So then it's up to the free/available time of the modlist maintainers (these days, Kulyok and me) to fix problems as they occur.