It is not just a serial cable. He is correct, there is a board built into it. If you take the housing off the serial end, here is what's inside:
There is a thread on here somwhere about building these. Someone was able to do it. There used to be a schematic floating around, but all of the links to it are dead.
Someday when I have time, I'll sit down with my cable and a multimeter and figure out the schematic again. I'll also try and dig up the thread and post a link to it when I have time.
Top of Equalink Cable board:
Bottom of Equalink Cable board: