I think he's modified the maps or something.
It was definitely new that he was able to shoot the Vortigaunt in the train car, which then angered the refugee that lived with him
It actually made me realize that scene is actually the first time you see a Vortigaunt in Half Life 2, and you're not told at any other point beforehand that they're now allies, so a player whose familiar with Half Life, would actually attempt to shoot them out of instinct
Oh well, it's not like Ross Freeman hasn't shot innocents before