My guess is that he thought doing a Roman salute would be wacky and edgy (and really popular with the cool kids on twitter), and that he could BTFO the lying media about it, but then he stuffed up the gesture horribly like the spastic he is.
I'm not a body language expert or a language expert but I have some sense of reading both. These were my observations during his speech, I invite anyone to add or detract from them with anything I may have missed.
He is incredibly excited. But he also wants a response from the crowd. We know he's looking for validation from the whole diablo/path of exile thing. He tries a couple of different things to get a response, some things work some things don't. At one time he's starting to talk loudly and he notices the crowd isn't with him. He calms down a bit to start building it back up. His words though are very telling. "This is what victory
feels like". His focus is on how this experience is going for him personally, what feelings it's giving him. Each time he tries to get a response with something physical whether slamming on the pulpit, or doing his spastic little dance, which would have been the highlight if he hadn't sieg heiled, you can see that he's uncoordinated with it, but also that it kinda overpowers him. All solo sportsman and women have the same problem that it's actually very hard to physically express that excitement on your own.
I think both your interpretations have merit. I think he means it wacky and edgy, but also as a nod to the irony bro nazis, not so different from 2015 /pol/ as
@Hollywood Hulk Hogan said.
In some ways his speech reminds me of Jeb Bush's "please clap". Elon is about as spastic as Jeb was pathetic. Both were approaching it from a needy position. Elon was really looking for that response, that's what his words and body language seems to tell me. And every response wasn't big enough so he kept trying to make it bigger. Like when he suggests planting the flag on another planet. Just doing it once isn't enough. He seems like he's completely carried away.
$$$
As for media manipulation, I already noted that conservative australian news put their logo exactly over his salute so you can barely see it. But Fox went further and edited it out:
