> Can anyone just sue anyone else for defamation?
Anyone can sue anyone for anything, but actionable defamation has a fairly narrow definition in the US, largely as a result of the First Amendment, and to survive the preliminary phases of legal action there must be at least an allegation with evidence proposed to be presented that could reasonably meet the definition.
> I'm just trying to figure out why Elon is in so much trouble over this.
Because people aren't, legally, free to publish harmful lies about others, and Musk stands credibly accused of doing that.
> He isn't an elected official or anything.
How is that even relevant? (Were he a public official, and the statement was related to his execution of public duties, he'd probably be in less, not more, legal danger because of governmental immunities.)
> Is he not allowed to call people names for some reason?
He didn’t just call someone names. He both publicly offered a bet that the label would prove to be factually accurate and followed the general accusation with detailed and specific supporting allegations of sexual predation on children, which were presented to a reporter.
> I once called my friend a douchebag, so can he sue me for defamation?
Since it is impossible for a human to be a literal douchebag, it is impossible for that to be construed as a fact claim. It is quite possible for a human to be a pedophile, and Musk’s conduct can (at a minimum) reasonably be seen as indicating that he meant the accusation to be treated as literal fact, not generic name calling.
> I have to agree with his lawyers here. I don't see a crime.
He's not accused of a crime; were he accused of a crime, it would be by a public prosecutor, not a private litigant. But if Musk's lawyer’s “Twitter is not for fact claims but only opinion puffery that no one takes seriously” argument had any real chance of being taken seriously by a court, he'd never have settled with the SEC (and then done so again when accused of breaking the first settlement) over his problem of tweeting about Tesla.
It's like Musk is dedicated to getting in trouble in every domain of law in which it is possible to get in trouble for improper fact claims, all by way of Twitter.