unless you know where theres some sort of enable multiplayer within the civ3 game itself, good luck finding the right place to "toggle". Assuming you can get a breakpoint, or whatever those special hooks in programs are, you'll be hard pressed to find the code. Also you'll only be looking at hex, not the real source code! Finally, decompiling the game and so forth isn't allowed
btw, look at the other text files, lots of them simply have text like stats, but also pop ups and message boxes. That's what jackal has, simple settings, warnings etc.
Remember that originally Firaxis said MP was in, so it's normal to see at least "some" MP related files. I would of removed it though
but maybe they left jackal.txt to keep the players optimistic. perhaps it's a hint that multiplayer will come for free (IMO infogrames aren't that bad ... ). At the very least though, traditional TBS multiplayer options should come for free, although I won't mind paying extra for their "revolutionary" TBS multiplayer stuff that was promised... how they will do that beats me though.