An interesting aspect is that not only the far-left is thinking it was a nazi salute, the far-right too, welcoming it and seeing it as Musk supporting them.
For me, the gesture is already quite suspicious (People have tried to cherry-pick similar gestures from other people, but they don't come close. The way the hand is flat at the end does not seem something that come "naturally" or "by accident", as it does not occur in others). But the reaction to the controversy is even more suspicious.
As I've said, far-right people were saying it was a nazi salute, and Musk never deny that to them. He criticized "far-left", or even instrumentalized it to attack people he does not like (for example, he attacked Wikipedia for just saying "some people compared the gesture to a nazi salute", which, according to him, is the proof that Wikipedia is part of the far-left conspiracy). He may have publicly said it was not a nazi salute, but it is clear that it is not enough to convince his far-right supporters it was not one.
So, yes, I think that "he has made a nazi salute" should be the default hypothesis. The data fits wayyyy better with this hypothesis than the one where it was an accident. No need to pretend that this hypothesis is favorised because of a conspiracy of biased people against Musk.
(Personally, I think that Musk did this gesture on purpose, in order to troll, trying to "trigger the woke". Does it make of him a Nazi? Well, some people supported the Nazi party because they were intrinsically "nazi" inside (whatever that means, probably something different for every single nazi). Some people supported the Nazi party just to troll their neighbour. They all ended up guilty of nazism all the same. So, while it can be an interesting nuance, it does not matter: if you are happy to play with the nazis, you are as toxic for humanity as them.)