You asked about drawing a line between speech and violence, not whether the supreme court's ruling is legally relevant today. The only relevant question is whether the argument the OP described clarifies your confusion on where and how to draw that line, ethically, not legally.

Ethically speaking, I think there are all sorts of things that just shouldn't be said, regardless of legality. I'm not sure if my personal sense of ethics is relevant though?

Firstly, "things that shouldn't be said" include matters of etiquette and not just ethics, so just be clear about the line between being a good/ethical person vs. being a polite person.

Secondly, your personal ethics are absolutely relevant because ethics advances by persuasion in rational debate. Everyone's views on such matters are relevant, if not persuasive.

Well, personally, if I attended or worked at MIT, I would not want Nazis on campus and would accept banning them.

Great, you're free to not want them, but everyone has to justify their actions, so an action like banning a group still requires justification. If that justification is sound and based on accepted ethical principles, it should arguably become an ethical rule. I personally haven't seen an argument that satisfies the requirements.

I don't think I've ever met a human who was incapable of justifying their actions.

They're all capable of attempting to justify their actions, but rarely is that justification sound. "Justification" here means a deductive argument from principles, sort of like what you'd find in a court of law based on except based on ethical rather than legal principles.