What disturbs me most is that once such a slander is spoken by the president of the USA it becomes prominently established in the public discourse and in many people’s consciousness as a possibility. The “where there is smoke there is fire” concept enters the average person’s brain even if there is no smoke but just a fat orange evil troll claiming he sees smoke. News sites favoring Trump run headlines, “President Trump reveals Buffalo protestor as Antifa crisis actor.” Mainstream news sites run headlines “Trump makes unsubstantiated claim that Buffalo protestor member of Antifa.” In either case the claim goes out widely. And even the term “unsubstantiated” doesn’t adequately deny the claim - it doesn’t negate it. Instead it simply suggests that adequate evidence has yet to be found. It leaves open the possibility that the claim might be true.
So Trump followers immediately accept the claim as 100% true. Many others think “hmm, I wonder maybe...”
I don’t know what would help. The term “baseless” better captures the utter emptiness of the claim than does “unsubstantiated” and I’ve seen that in some headlines. Perhaps even better the headlines should read, “In a completely fabricated, vicious slander of a seriously injured fellow human being, Trump claims...”