Yes it's a mod. My point I suppose is that MP support should have been added in by ETS devs, especially for ATS. The modders are stuck with what they have to work with and it's a pity because ETS/ATS would massively benefit from working MP.
to be honest the main problem with multiplayer games is other people just have to ruin it.
ETS2 is a fantastic game BECAUSE it is pure single player imo. i can just imagine now how it would be if it was multiplayer.... people trying to take it semi seriously having to put up with berks ramming and doing all sorts of stupid stuff.
having it as a fan made mod bolt on is a fine compromise imo.... .and because people have to go to some effort to set it up may ironically mean you get a better type of gamer playing the multiplayer as well. (maybe not i dunno, i have not tried it MP)