Hitchen's Razor applies - 'That which is asserted without evidence can safely be dismissed without evidence.'
The burden of proof lies upon the person asserting a statement as true. There is no burden on others to prove a statement to be false.
I should add that a poll is not proof, it's just opinions. Worse still, depending where a poll is published and how the question was worded, it could lie anywhere along a spectrum:
biased against<--neutral-->biased towards.
But why do I have to keep repeating the fact that Mussolini was the originator of the modern Fascist salute (saluto romano) in 1919 with it becoming the standard fascist salute by 1923. Hitler merely copied the fascist salute in 1933. All Nazis are fascists but not all fascists are Nazis. Neither are based on the Roman salute since it's not known what form of salute the Romans actually used. But if people have forgotten, The Roman Empire was an oligarchy that decended into a dictatorship - so asserting it was a Roman salute doesn't really make things any better.
I feel that at best it was an equivocal gesture. People were free to read it as they wished. That may well have been calculated.
What I find telling is that Elon Musk has very carefully refrained from simply saying something along the lines of 'I had an on-the-spot idea of saluting the audience but realize in retrospect that the gesture might have been misconstued.' Read that again - it's still weasel words but would likely appease the media.
Why? Why not do that one simple thing? Becuase it BENEFITS him not to.
Some possible reasons:
Because he IS a fascist (signalling true political position/political gain)
Because it bugs some people (antisocoal personality/trolling)
Because he is incapable of admitting to his mistakes (narccism)
Because it drives traffic through his faiing social media platform (corrupt practice?)
Because fascists in the USA are a voting demographic and WILL believe it to be a fascist salute (narccism/political gain)
Because it keeps his name in the media and he doesn't care if it's for a good or bad reason (narcicism)
I say this because more often then not, people do things for multiple reasons. None of us are privty to Elon Musk's internal life so we cannot be certain which if any of the above are true or if additional or alternative reasons were involved.
That said, none of the above would show him in a good light. Not a stable personality at all. I posit that any rational person would be at pains to clear up any confusion as soon as possible. But we are living in a post truth age.
Still - I don't think there is a poll. I spent quite a long time searching for it with no luck. So I should say 'the weight of evidence suggests no such poll exists'. Maybe someone just dreamed it? I don't know.