Mass hysteria strikes me as even more plausible if you consider that some of the people reporting symptoms are American intelligence agents who could be participating in a disinformation campaign to give a pretext to act against Cuba.
Then we blame the Russians while tut tutting the Cubans for allowing this to happen.
That’s the rather tidy thing about claiming to be attacked by a weapon no one can see and that leaves no evidence of use: you’re free to build theories that suit your geopolitical ambitions.
A tut tut isn't the same thing as an "act against Cuba."
If you believe that there are several diplomats with real injuries and persistent symptoms, then it stands to reason that something happened. No amount of FUD can controvert that.
Personally I have no geopolitical ambitions. What I do have is a great deal of faith in the American investigative apparatus, and none whatsoever in the honesty of Russia and China.