You are quite right that if both had ADS-B on board, both side may have "see and avoid". If my memory is correct, in this case one was about to land and the other about to take off in poor visibility. None could have time to see the other and avoid. Yes all use mountain frequency when flying in that area, but people are usually on different tone. The French speak French and the Italians speak Italian. A/A communication has always been a problem there. A scapegoat has to be found, thus the flightplan.