The US has a very high standard for "defamation" against "public figures" and a very low barrier to be considered a "public figure". By US standards, the diver was a "public figure", and Musk's insulting comment was not defamation, because it was hyperbolic (comparable to when Hustler Magazine published a fake ad about Jerry Falwell having sex with his mother in an outhouse).
Bottom line is that it's very hard to win defamation cases in the US. The Tesla engineer might have a shot because she was not well known, and Tesla accused her of very specific acts (moonlighting, unapproved travel) and they knew those accusations to be false. On the other hand, these were not exactly crimes, so who knows.
The legal system has a concept of "no harm no foul," (i.e. the concept of a "tort"), and if nobody believed Elon's obviously stupid tweet, there would be no tort to sue on.
I never could understand how what he said wasn't considered defamation. If that's not defamation, what is?