Elon Musk couldn't (or, most certainly, shouldn't) have been found to have defamed Vernon Unsworth by an English jury either.
If I refer to a fellow ABer as a "childmolesting, kitten-strangling, drunken, narcissistic sado-masochist" on this forum, that is most definitely NOT 'defamation', as defined by the Defamation Act 2013.
That's because the Act states "A statement is not defamatory unless its publication has caused or is likely to cause serious harm to the reputation of the claimant". Since the person I had referred to would be unlikely to go down in the opinion of his family, the people he works with, the people he socialises with or other AB members (who who almost certainly recognise that, while my post was offensive, it wasn't true), the test for 'defamation' would not have been passed. Making grossly offensive statements about someone is NOT the same as 'defamation'.
In an English court the onus would have been upon Mr Unsworth to show that his reputation had suffered serious harm (e.g. by showing that he'd lost work through people believing the accusation). If he couldn't produce such evidence, an action for defamation would automatically have been doomed to failure. Saying (or even proving) that Mr Musk's statement was untrue and/or that it was grossly offensive would be completely irrelevant.