For not existing in any organized way they somehow manage to gather in large groups, coordinate their clothing, have local chapters, have flags, banners, t-shirts and hoodies made, destroy and burn cities in coordinated groups.
If antifa is just a noble and just concept then you shouldn't have any problem declaring yourself one of them.
Ironically, those who argue its just a concept certainly exhibit the characteristics of a fascist more than the people they allege to oppose. They silence, disrupt and beat those who disagree with them.
On a serious note, it seems antifa and the murderous assassin Robinson share a critically core belief: beliefs which oppose their own are dangerous hate. Its actually a common core heuristic for most on the left.
If its nothing, who are all those people marching with the antifa flags and banners attacking people, throwing things, fighting people, burning things etc in the above video? If its not a thing, who are those people?