He's probably running Aegis if its true? But the last time I checked and afaik DarkRO has the largest pop back then.
Bandwidth consumption is low indeed and it is possible to have a server with 30,000 concurrent connection. Only thing you need to do is edit Linux headers to allow more than 1024 connection otherwise it wont happen.
On a side note:
There is a server last 2012 with more than 24k players. It's a Thai server running in Eathena.