You are right that fascism cannot exist without authoritarianism, as fascism is authoritarian in its very nature. But you can absolutely be authoritarian in nature without being fascist, as is the case with communism, which is inherently authoritarian but diametrically opposed to fascism.
You are right that ultranationalism is a key tenet of fascism that is missing in antifa, but you are wrong to say it’s the only one.
Fascism is also defined by a strong leaning towards tradition, and a return to a better, more pure time in the past. This is not a feature of antifa, who if anything have a desire to upend previous values and social structures in the name of what they see as progress. I think it’s fair to say that people in antifa reject tradition, if anything.
Corporatism is usually a feature of fascism, and is based on the belief that social classes do not conflict with one another, but can exist in harmony for the benefit of the greater good and national interest. People in antifa are definitely far more likely to take a more socialist perspective on this, recognising the antagonism between different social classes and the subjugation of those in lower social classes at the hands of big business, or the bourgeoisie, to use the socialist terminology.
Hitler said “victory is to the strong and the weak must go to the wall”, because a core trait of fascism is social darwinism, the belief that the disabled, weak or morally degenerate are a poison to the health of the nation or group. Again, this is absolutely not a trait that antifa members would share. Fascists forcibly sterilised the mentally and physically handicapped, and members of certain ethnic or religious groups, in the name of protecting the nation. People in antifa dont go in for this kind of eugenics, they’ll typically reject social darwinism and believe that “the weak” should be protected by the rest of society, not exterminated. Fascists also have a big issue with sexual degeneracy, as they’d call it, and would persecute these groups, possibly to the extent of systematically murdering them if “lesser” discriminatory measures didn’t do the trick. People in Antifa very strongly reject this, and in fact go out of their way to strongly advocate for the support of queer and trans people.
I could spend more time talking about how the comparisons between fascists and antifa in terms of an obsession about race is facile, but I think I’ve already done enough to demonstrate that antifa clearly is not fascist, because it misses many of the defining characteristics (not just one). Besides, while I am off work, I really should make an effort to get some shit done today lol, as much as I enjoy a good political discussion.