I just don't know what a 'ROM game command' is. Seems more likely it would be controller inputs to one person's emulator? Or perhaps everyone has an emulator and the inputs are simply shared.

To me it seems like just a somewhat awkward way to phrase exactly what you think it means.

It’s probably really simple: if you normally load a ROM game and start a multiplayer game you control all the other players input. This simply goes next step and makes other inputs be controlled by other players.

These things usually work by syncing the emulator clocks between clients, and then sending player inputs over the network along with the data to sync.