25467 is a valid port number.
You're sort of wrong about the slots thing. There's 15 total servers listed, 12 of them are online, and 9 of them are Capture the Flag, with the rest likely being Free for All (way more common for online shooters originally) or Team Deathmatch (much more common for more modern shooters), and are simply being filtered out. The weird part of this server list is normally, especially the way the servers are named, they wouldn't be preloaded by the game in question, which is the only way you'd see an offline server in the first place. Usually, the 12/15 means that you're viewing the 12 servers that are filtered out based on your settings, which in this instance we are forced into Capture the Flag.
This is likely the case here as well since there are only 9 servers listed, with 6 of them online. Another oddity with the server list is that one of the servers is LAN, which most games from the early 90s you had to search for using a specific filter, and wouldn't be preloaded into a server list like this, even for games that had that kind of set up (I can't think of one that used a pre-programmed list, or even showed offline servers LAN or otherwise for that matter, and I've played every multiplayer Quake, Doom, and even a few shooters that weren't that well known). Most games that did use a pre-programmed server list didn't show you the server list, and weren't shooters.