That would be news to me. Mal, as he said in the
wiki and sticky
thread before he left to work at id, did not have a Mac nor did he have experience compiling on a Mac. But you only need to have the server running windows. Any Mac running standard ET can then connect to that server by LAN etc. and play with the bots. Ditto for Linux.
But if you have only one machine then it needs to run Windows.
Now the server does not have to have some great video card as it can run as a dedicated server rather than a host. And you could use a null modem Ethernet cable if you don't have a hub/switch or router to connect the computers (If you can connect by wireless great too.) Just be careful not to mix up the null modem cables with the regular ones.
So for a few bucks get a second hand Windows computer and teach yourself some networking basics I say.