Sure.... did you have a tape of him admitting it in a serious truthful manner, otherwise the rest is less than circumstantial and is more likely cosplay.
Are you saying Cosplay is not Satanic? I would say dressing up as a character in a movie, video game or some other form of media is a form of idolatry.
We are not talking about a documentary where the actor is portraying a real life character, we are talking about fictional characters and fantasy. To dress up as this character, as Elon Musk did, involves an investment, that is an offering to that idol. His purchase of that outfit and wearing it constitutes an offering to an idol and worship of that idol. Since that idol portrayed Satan, by definition it involved making a sacrifice to Satan and worshipping Satan, hence a Satanist.
Many of these characters in Cosplay are considered false gods (Thor, Loki), types of the Antichrist (Superman, etc), demon possessed (Gollum, etc). I agree to infer that someone wearing a costume of Thor is "Satanic" is an inferential leap that many would not make. But to dress up as Satan is not a leap in any sense of the word.